Why was I Christian? because my mother was. She was Baptist before JW. I'm a born-in. Once I started seeing the world outside of JWism, I explored other Christian sects. None made sense. I am agnostic Deist and no longer Christian - I believe in God/dess but I admit that I cannot prove to an atheist that She exists.
I agree with a PP that said many of us were/are Christian because of where we were born. If I was of Asian descent, I would most likely be Buddhist from birth.