In your opinion, what keeps them in?
At the very real possibility of being called a broken record, please indulge me. . .
Have you read 'Combatting Cult Mind Control' by Steve Hassan?
Most JWs are just like David Koresh's followers, prior to that disaster. They were incapable of leaving. They were "captive to the concept" that David Koresh was God's prophet. They would rather die than leave God's prophet.
Watchtower is not simply just another church, it's a high-control group, in which a subtle and all-invasive system of thought reform is present.
I will remind you and others. There is (essentially) only one reason that I escaped: A false positive blood test that led my wife's doctor to tell me that my unborn son would need an injection of reb blood cells. Had this one event not occurred, I would probably still be a JW. It was completely random.
They were controlling me 100%, up until the moment I snapped and began investigating the religion with a critical mind (as my heart raced with fear of apostates and demons).