I kind of feel that, as a teenager as I started to read the Bible more I found a lot of the things that Yahweh did or that prophets did in his name by using their powers, or some things in Psalms that were praised (like saying they should kill Babylonian babies by hitting them against rocks) to be really horrible and gravely offend my innate moral compass. And I realized that any permanent punishment for any temporal crimes against an infinite unhurtable god was not justifiable and so despite the nicer qualities of Jesus I couldn't even get behind him for how much he talked of hellfire. That was the starting point. Then I found a bunch of weird claims that couldn't be true. And I failed to find any solution to resolving the very evident age of the Universe with the claims of the Bible. The last nail in the coffin was that I tried it out and prayed a lot but nothing happened, but the moment I turned to another religion and prayed stuff happened very quickly and the experience was very real and vivid.
I think though the underpinning reason for me was that every denomination I've ever known, heard about, or whatever didn't actually follow the Bible very accurately. And when I actually read it for myself it became apparent to me that it would actually be very scary if anyone did follow it, as the things it would have people do are pretty violent at times.
Another thing I should point out is that my current understanding is more mature of the Bible. The ancient Jews wanted to preserve the traditional teachings and so they combined different versions of the same stories which is part of why the Bible is full of so many contradictions. This becomes more clear when you realize that in many parts of the Old Testament it tells the same story more than once, it's actually telling it again as the different version. They wanted to preserve all the versions into a single set of scriptures to unite their people. I'm pretty sure everyone who read it back then knew that it was supposed to be like that, and so wouldn't see them as contradictions but more akin to disagreements, perhaps.
Though the New Testament is a bit' different, because the context was way different. There were hundreds of gospels written all wildly different but only a few survived the gnostics' suppression and were later chosen to be part of the canon. The four gospels of Jesus Christ are more consistent than the old testament, but some things like Judas's death stand out. In one account he accidentally slips on a sharp rock and spills his guts and the guys take his money and do something with it. In another Judas feels bad and commits suicide and the guys do something different with his money.
In either case, if a scripture can't even agree with itself which version is the "real" one I can't see it as being the "Inspired Word of God" as I kind of think if there is a God, an intelligent creator of the Universe who planned this all out, he would of been better at getting people to write the truth down correctly and consistently.
Though I guess you could always take just one version of each story as the "true" one but then you would disagree with pretty much nearly all Christians and you really wouldn't have the Bible anymore but something lot shorter.
While I am no Christian, clearly you have never read the entertaining and sometimes hilarious parts of the Old Testament. I could give some examples if you like.