1) They don't see their parents practicing the faith. The parents go to church, sing some songs, say amen every two minutes, and then go home and act secular and worldly again.
2) They don't actually learn the Bible (cough cough the charismatic church cough cough).
3) They are taught false Christianity.
4) They are taught a legalistic Christianity. Parents force their kids to read the Bible, instead of setting the example and encouraging them to read the Scriptures.
5) They have never heard the Gospel. I only heard the actual Gospel after I was saved (guess I wasn't saved then either). My youth pastor and my head pastor do not tell their congregation that they are sinners and that Jesus died for their sins. They only say that God is gonna bless us materially. While that MIGHT be true, that is not the Gospel, and that is not going to convict anyone, and that is not true biblical doctrine. Kids need to hear the Gospel, now more than ever.
6) Unbelief.
I understand that this is not speaking for all Christians, but these are some reasons why our kids are apostatizing.