Reason for rise in evangelical observance of Lent in the United States I grew up and continue to live in the United States.  Maybe it's just me, but I was always been under the impression that Ash Wednesday and Lent were celebrated by Catholics and not really by anyone else.  Apparently I was wrong and some Protestant denominations do and have always celebrated it.  Even so, it does seem like within the low-church denominations (e.g. Baptists, Methodists, Pentecostals) the practice has always been eschewed.  Again, maybe I'm wrong, but it just seems like evangelicals observing these holy days has been becoming more and more popular.  Firstly, am I mistaken?  Secondly, if I'm not mistaken, is there some documented reason for this?
While observance of lent is definitely on the rise; this is especially true of Denominations closely related to the Catholic and Orthodox Denominations, as is commonly known both in the Christian and non Christian societies. I can only speak for my Southern Baptist faith.

In most of our churches fasting is being more emphasized, as a strengthening factor for prayer. And that precept is based on:


  Matthew 17:20 and 21 NKJV  So Jesus said to them, "Because of your unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, 'Move from here to there,' and it will move; and nothing will be impossible for you. 21  However, this kind does not go out except by prayer and fasting."


and:


  Mark 9:25 through 29 NKJV  When Jesus saw that the people came running together, He rebuked the unclean spirit, saying to it, "Deaf and dumb spirit, I command you, come out of him and enter him no more!" 26  Then the spirit cried out, convulsed him greatly, and came out of him. And he became as one dead, so that many said, "He is dead." 27  But Jesus took him by the hand and lifted him up, and he arose. 28  And when He had come into the house, His disciples asked Him privately, "Why could we not cast it out?" 29  So He said to them, "This kind can come out by nothing but prayer and fasting."


My personal opinion is that as the World as a whole becomes more and more evil, while Christian attitude among the general public wanes, and God is increasingly being removed from Society more and more Christians are adding fasting to their prayer vigil. This is the prevalent attitude among my co-members.