I'm of the opinion that most who leave don't leave because they become convinced it's not the truth. That's just based on my own personal experience. Can't say whether that's the case today or whether it's the same in every region of the world.
Most seem to leave because (a) they give up because they feel they're not strong enough to live up to the religion's standards or (b) because they get into some personal issues with other JWs and can't bring themselves to remain part of the community.
The problem is that people in those situations live their lives with constant guilt. It's incredibly damaging from a psychological perspective. They think God is good and showed them the road map to everlasting life, but they're too selfish to obey. Or that they're letting their petty issues with others stand in the way of doing what God wants.
A few younger ones convinced themselves it was okay to stray away for a while, live a "sinful" life, and come back a few years later when they're ready to settle down.
These type of people usually come back at some point. Some make multiple failed efforts to return and remain in "good standing."
In my local area I can only think of two others who left for reasons of conscience. There are dozens, however, who fit the mold of what I was describing earlier.
I hope the internet is helping change that, but I've been away from all things JW for over a decade.