Hmm, well, for me it's a long story, but you asked for reasons so I figured I'd give details
Grew up Evangelical Christian, although at about age 13 or so I read one of those tracts and it convinced me I wasn't a "real" Christian so surely I was going to hell. Did the whole ask Jesus into your heart thing, and thought yay, I'm a real Christian now! I really got into it, too.
Over the years there were just so many problems with it, though, the biggest one being the issue of hell, and so I ended up drifting away from most of my faith although I still called myself a Christian.
I finally decided I wasn't one anymore, after days and days agonizing over it and reading different arguments. Part of this was due to the influence of someone I met who was the first to really challenge my beliefs.
It took a while but I slowly became more confident in my decision after being on the fence for a time.
Now I consider myself agnostic/atheist. I don't like hardcore materialists - I find
them to be too closed-minded for my liking. I like the idea that there
could be something out there, something we've never dreamed of. Not necessarily a god. The universe is a vast and strange place and I am of the belief that we know much less than we think we do. But then, who knows?