What is meant by keeping people blind and deaf so as not to get healed? What is meant by keeping people blind and deaf so as not to get healed? [see footnote regarding answers]

I understand this to be one of those scriptures that is meant to challenge our understanding, such as "drink my blood and eat my flesh"...yet I still can't see the purpose behind these words:

Isaiah 6:9,10


  “‘Be ever hearing, but never understanding;
      be ever seeing, but never perceiving.’ 10, Make the heart of this people calloused;
      make their ears dull
      and close their eyes.[a] Otherwise they might see with their eyes,
      hear with their ears,
      understand with their hearts, and turn and be healed.”


Mark 4:10-13


  10 When he was alone, the Twelve and the others around him asked him
  about the parables. 11 He told them, “The secret of the kingdom of God
  has been given to you. But to those on the outside everything is said
  in parables 12 so that,
  
  “‘they may be ever seeing but never perceiving,
      and ever hearing but never understanding; otherwise they might turn and be forgiven!’[a]”
  
  13 Then Jesus said to them, “Don’t you understand this parable? How
  then will you understand any parable?


What is meant by keeping people blind and deaf so as not to get healed? 

edit: 

I've reopened the bounty. The entire bible is encoded in parables, and Isaiah 6 eludes to what the purpose of encryption-through-symbolism is needed in the word. Jesus applies it, but does not explain it's full necessity, and why it must be that way. 

So we ask again - what is the meaning of "Keeping people blind and deaf so as not to get healed?" a concept which contradicts the idea "I want to, be made clean...". 

In spirit this concept contradicts the nature of God, who wants to heal and restore. Yet this says "Make it so that cannot happen". To a Christian, one would be confused the same as Jesus Jewish audience when he said "Drink my blood"...something that seemingly contradicted their law. 

Hence we seek to explain the mystery and remove the supposed contradiction. 

This mystery is so important to me I'd happily keep reopening the bounty, using up my points, until a conclusively accurate answer is given. 

FOOTENOTE:

Of the answers given, three authors gave appropriate and correct answers from different angles, thus giving a very good understanding of the possible answer to this question. 



@ShemSeger's answer was the first comprehensive one given, helping to understand the purpose behind the veiling of scriptures as it pertains to our human accountability. See the first paragraph see his heading "Understanding Parables". This was auto-awarded a bounty due to votes because I had sought more explanation on the issue. This answer, though, certainly gives a glimpse into what it means for our personal welfare.

@Trober's answer gives a very in-depth look at how this mystery pertains to God's justice and mercy, essentially giving everyone a chance to benefit equally at the right time from God's efforts to save and heal humankind. His answer addresses the answer from a prophetic aspect, citing very, very good examples in the scriptures to which this question pertains.

@Andrew's answer relies completely in the scriptures, and in it's own right very complete and elegant. It would have been assigned the "bounty" points, except many who are not trained in the scriptures might not see how these scriptures relate, although his outline is very authoritative. 

Thus Trober's answer is given the bounty for the most harmonious and well explained answer. ShemSeger's answer elaborates greatly on our own responsibility, and its meaning to individuals. Andrew's answer solidifies these things with well chosen supporting scriptures. 

The complementary nature of these answers reminds me of this scripture: 1 Corinthians 12:12-31
TL;DR: Milk before meat. Deep doctrine is dangerous to an unprepared soul. 


  "I have fed you with milk, and not with meat: for hitherto ye were not
  able to bear it, neither yet now are ye able." (see 1 Corinthians 3:1-3)


Christ taught in parables [as did Isaiah] so that the  spiritually immature and spiritually mature could both benefit from the message. He kept the deeper doctrines hidden in the parable from those who were spiritually "blind" and "deaf" to them until they matured spiritually enough that they could bear the deeper doctrines, and be healed by them.



Understanding Isaiah

You are right that this is one of those scriptures that is meant to challenge our understanding. Prophets like Isaiah veiled their teachings in language that preserved their teachings for those who would understand with their heart. Isaiah’s words are similar to the parables of Jesus in their manner of teaching. Spiritually speaking, Isaiah’s writings are meat, not milk (see 1 Corinthians 3:1–3; Hebrews 5:13–14; Isaiah 28:9). It requires spiritual maturity to understand them.



Prophecy of the Rejection of Spiritual Things

Isaiah 6 is a prophecy of the Jews rejecting Christ's teachings. The words the prophet Isaiah was commissioned to deliver were in part to bring the people to a full accountability for their choices, so that they would be left without excuse. The words are better understood:


  "And he said: Go and tell this people—Hear ye indeed, but [they]
  understood not; and see ye indeed, but [they] perceived not."


The people claimed to hear and see, but they did not understand the spirit of the message.

–– UPDATE – in response to OP edit:


  "Make the heart of this people fat, and make their ears heavy, and
  shut their eyes; lest they see with their eyes, and hear with their
  ears, and understand with their heart, and convert, and be healed."
  (Isaiah 6:10)


Keil and Delitzsch, Commentary on Isaiah 6:


  The command to “make the heart of this people fat, … their ears
  heavy, and shut their eyes” is used to describe the process of making
  the people accountable. The command, of course, refers to “their
  spiritual sight, spiritual hearing, and spiritual feeling.” (Keil and
  Delitzsch, Commentary, 7:1:200).
  
  “There is a self-hardening in evil. … Sin from its very nature bears
  its own punishment. … An evil act in itself is the result of
  self-determination proceeding from a man’s own will.” (Keil and
  Delitzsch, Commentary, 7:1:201).


An individual cannot resist or reject the truth without eventually becoming spiritually hardened. 

Isaiah’s indictment of the kingdom of Judah was cited again in the New Testament to show that the people of that time were no different. The inability of many to understand the parables is a fulfillment of Isaiah’s prophecy:


  "And his disciples asked him, saying, What might this parable be? And he said, Unto you it is given to know the mysteries of the kingdom of God: but to others in parables; that seeing they might not see, and hearing they might not understand." (see Luke 8:9–10; also see Matthew 13:10–17).


The significance of many of the miracles was also misunderstood:


  "He hath blinded their eyes, and hardened their heart; that they
  should not see with their eyes, nor understand with their heart, and
  be converted, and I should heal them." (see John 12:37–41). 


The testimony of the Messiah and His Sonship was understood, at least in part, by the disciples, but it was rejected by others:


  "21 ¶In that hour Jesus rejoiced in spirit, and said, I thank thee, O
  Father, Lord of heaven and earth, that thou hast hid these things from
  the wise and prudent, and hast revealed them unto babes: even so,
  Father; for so it seemed good in thy sight.
  
  22 All things are delivered to me of my Father: and no man knoweth
  who the Son is, but the Father; and who the Father is, but the Son,
  and he to whom the Son will reveal him.
  
  23 ¶And he turned him unto his disciples, and said privately, Blessed
  are the eyes which see the things that ye see:
  
  24 For I tell you, that many prophets and kings have desired to see
  those things which ye see, and have not seen them; and to hear those
  things which ye hear, and have not heard them." (see Luke 10:21–24).


In direct response to your comment:

"... it is mentioned that people should be deaf and blind until the cities crash down and the human population is decimated."


  "Then said I, Lord, how long? And he answered, Until the cities be
  wasted without inhabitant, and the houses without man, and the land be
  utterly desolate," (Isaiah 6:11).


The prophet Isaiah asked the Lord how long some men would be hardened against truth (v. 11); the answer—until mortal man no longer exists.

––



Understanding Parables

These scriptures refer to spiritual understanding. Many of the people of Jesus’ time were spiritually immature and unprepared to receive the doctrines He taught (they have eyes, but they can't "see" the deeper meaning). Through parables He was able to teach the more spiritually mature and at the same time veil His teachings from those who were not prepared to understand or follow them. In that manner He kept many from being condemned for having a knowledge of principles they were unable to live. 


  "That seeing they may see, and not perceive; and hearing they may hear,
  and not understand [hear the message, but not understand the deeper spiritual context]; lest at any time they should be converted, and
  their sins should be forgiven them [so that they can can join the church later, without being held accountable for sinning while having a greater understanding of spiritual things]." (Mark 4:12; KJV)


Matthew's account gives us more clarity on his words:


  11 He answered and said unto them, Because it is given unto you to
  know the mysteries of the kingdom of heaven, but to them it is not
  given...
  
  13 Therefore speak I to them in parables: because they seeing see
  not; and hearing they hear not, neither do they understand...
  
  15 For this people’s heart is waxed gross, and their ears are dull of
  hearing, and their eyes they have closed; lest at any time they should
  see with their eyes, and hear with their ears, and should understand
  with their heart, and should be converted, and I should heal them.
  
  16 But blessed are your eyes, for they see: and your ears, for they
  hear. (Matthew 13:11,13,15-16)


In answer to your question, the people cannot be healed until their spiritual eyes are opened, and they can hear with their spiritual ears. Only then can they understand with their heart. Therefore it is prudent to keep them blind and deaf to the deeper "meatier" doctrines until they are spiritually mature enough to "stomach" them, Otherwise it could be detrimental to their spiritual development, much like giving meat instead of milk to a suckling babe would be detrimental to the physical health of the baby.

The Savior’s parables therefore included an implied invitation to seek further understanding of the truths they contained, and those who did so were rewarded with greater enlightenment. 

A simple illustration showing why some people may fail to understand the truths taught in parables: 


  “Two men may hear the same words; one of them listens in indolence and
  indifference, the other with active mind intent on learning all that
  the words can possibly convey; and, having heard, the diligent man
  goes straightway to do the things commended to him, while the careless
  one neglects and forgets. The one is wise, the other foolish; the one
  has heard to his eternal profit, the other to his everlasting
  condemnation” (Jesus the Christ, 3rd ed. [1916], 297).




The Establishment of Zion (Isaiah 1–12), (13-37).
Old Testament Student Manual Kings-Malachi, (1982), 137–50.Understanding Isaiah, (E-3)
Old Testament Student Manual Kings-Malachi, (1982), 131–35.Chapter 5: Matthew 13–15, Matthew 13:13. “They Seeing See Not”
New Testament Student Manual, 2014.