Why are Mormon Temples Ceremonies kept secret from non-Mormons? Even though only priests were allowed into the Temple in the Old Testament, all of the ceremonies and furnishings were described in great detail in the Old Testament for anyone, Jew or Gentile alike, to read and understand.

However the details of Mormon Temple Ceremonies are kept secret, and, in fact, non-Mormons are prevented from even entering Mormon Temples. The LDS website says


  You can talk about what the interior of the temple looks like, and you can freely share the feelings you have in the temple. However, temple covenants and ordinances, including the words used, are too sacred to be discussed in detail outside the temple. By avoiding discussion of these sacred things outside the temple, we protect them from mocking, ridicule, or disrespect. Do not be casual when talking about your experiences in the temple. (lds.org)


What is the reason for keep the ceremonies secret? Why are they considered 'too sacred'?
There seems to be two questions here.  First, "why are LDS temple ceremonies kept secret, seeing as how Old Testament temple ceremonies were not?"

The simple answer is that the Old Testament temples operated under the Law of Moses and were administered by the Levitical priesthood, whereas modern temples operate under the Law of the Gospel and are administered by the Melchizedek priesthood, which necessarily means that things will work differently.  (See Hebrews chapter 7, particularly verse 12.)  Therefore, it's unreasonable to expect that the Old Testament temple ceremonies--many of which had to do with matters of blood sacrifice and similar rituals of the Law of Moses, which were fulfilled in Christ--should be perpetuated in modern temples.

Second, the slightly broader question, "why are LDS temple ceremonies kept secret at all?"

The answer to this is somewhat subtle, and some people will probably find it unsatisfying, but the Church does not hold them to be secret at all, but rather it considers them sacred.

The value of a secret is exclusivity.  The less people who know about a secret, the greater the power it confers upon those who know it.  Throughout history, people have been known to go to great lengths to keep secrets, up to and including committing murder to prevent (or retaliate for) the sharing of a secret.

By contrast, the Latter-Day Saints do not see the temple ceremonies as something secret and exclusive at all, but rather as something that is highly sacred.  The church has spent a great deal of effort, time and money in both building temples throughout the world, and sending missionaries to teach people the Gospel, with the explicit intended goal of bringing them to the temple.  This is the exact opposite of a secret: it is something that the Latter-Day Saints put a great deal of work into sharing as broadly as possible!

But because these matters are viewed as sacred and highly spiritual, that sharing is done within strict boundaries: only people with a certain degree of spiritual preparation are allowed to participate.  This is quite in harmony with Paul's teaching on the Lord's Supper:


  1 Corinthians 11:27-29
  
  27 Wherefore whosoever shall eat this bread, and drink this cup of the
  Lord, unworthily, shall be guilty of the body and blood of the Lord.
  
  28 But let a man examine himself, and so let him eat of that bread,
  and drink of that cup.
  
  29 For he that eateth and drinketh unworthily, eateth and drinketh
  damnation to himself, not discerning the Lord's body.


As the purpose of the temple ceremonies is to help people along the path to salvation, and not to help condemn them, the church requires that certain standards of preparation and worthiness be met before participating in these sacred ceremonies.