I think because they don't want to follow the rules. Teenagers like to be free, party and meet boys and girl; they want to have fun! Who can blame them, there young. But, the bible says "Train up a child in the way he or she should go and he will never depart from it. So if there parents taught them well, they may run from it now, but they will come back to it later.
My name is Jacob and I graduated from a college in divinity and pastoral theology. I believe the number one reason that teenagers are turning away from Christianity is because of the lifes their parents lead who say they are Christians. Too many parents are asking their children to be great people of the faith without they themselves being great people of the faith. Secondly, the reason most teenagers are turning away from Christianity is that they see the rediculous practices of the preachers and elders of the church. I am not at all saying that all preachers are this way. Yet, by experience I have just seen too many preachers being caught up in themselves and not their people whome God gave to them to minister to. I hope and pray that this helps you.
Because of peer pressure to do wrong by unsaved friends and family
Well I strongly believe in the influence their frens have on them too
Because of bad influences
Because they are taught science in school and see following a book written by many PEOPLE, NOT God is basically like following adult children's stories and have decided to use the senses we were born with and actually study the world around us and follow what science shows is actually right in front of us, not what some magical huge being in space wants us to do