It was a slow accumulation of unanswered questions over a number of years. For years, I just kept pushing these issues aside, using thought-stopping techniques such as "where else would I go?"
For better or worse, I have a logical mind and little tolerance for B.S. Not surprisingly, this made swallowing the anti-science, anti-progress, bigoted fundamentalism dished out at the Kingdom Hall quite challenging at times. Eventually, the cognitive dissonance became too much, and I decided to really dig into the Bible. The main issue to me was the faithful and discreet slave. Specifically, does that parable give the Watchtower Society the power to make rules (over and above what is set forth in the Bible) for Christians?
I simply dug into the Bible itself, reading the scriptures in context. The more I read, the more I realized that the Watchtower Society's authority structure was built on a house of cards. When I mentioned some of my concerns to Witness associates, they recoiled in fear, using the "apostate" epithet. This further convinced me that I was on to something. Why would members of a so-called Christian group have such a visceral reaction to a fellow believer digging into the scriptures? It reminded me of the Catholic Church of the Middle Ages.
So, to answer the question posed, I suppose the reason I left the organization was that, from reading the Bible and WTS publications, I became convinced that it did not represent the "faithful and discreet slave" of Matt. 24 and Luke 12. In fact, I realized that the FDS doctrine was a transparent power grab.