1.) I think many have not joined other churches because at first they may have felt "religioned out." The JW routine takes so much out of a person, the idea of being required to attend another institution seems sort of repulsive. Generalizing here.
Then sometimes sooner or later, the desire to meet up with a group for worship sounds appealing. Or some find spiritual fulfillment in other ways like spending quality time with their family, volunteering, focusing on self improvement...whatever. Just my opinion.
2.) If you look at the Bible honestly, you know there are contradictions galore. Then comes the choice to ignore the inequities and rely on faith in the "good book" no matter what, or investigate the Bible itself. It's really all about faith.
There have been fascinating discussions here between believers and non-believers. If you take creation, no two Christian faiths believe exactly the same about it. The WTBS is distancing itself from their original doctrine regarding creation. As far as the flood, well, if you read the science, or even just think about what would have to happen in a global deluge, it would spark some doubts. But if you want to have faith the flood happened just as the Bible says, no matter what scientists say, then again...it is nothing more than a faith issue.
Yes, it usually starts with disappointment in JW. But when you realize they didn't come up with their doctrine, most of it came from elsewhere, you begin to see JW's are just a speck in the world and in the stream of time. Some have had horrible experiences within the JW, and that would put one off from anything JW, or even religious.
A Christian is defined as a follower of Christ. There can be quite a variety between Christians of what a core doctrine is of the Bible. I see some don't agree on what one has to do to be "saved." That in and of itself, for me, leads me to question a god who wants no one to die, and everyone to live, who makes it so vague as to what he requires to do so. That's just me.
3.) I haven't a clue.
Good luck on your journey.