It's because of how hard they are on their members. As simple as that.
If you have a bully in school that picks on a ton of people, most of those people when they get a chance will get back at him or not help him out in his time of need.
The same is with this bully of an organization. They crap on their members, harsh and make them feel bad about themselves. Instead of labeling everyone who disagrees as an apostate and actually took a minute to see why someone was upset or didn't believe a teaching I doubt people would be as hard back.
They use and abuse you until the 2nd something goes wrong in your life. Then, you're casted out and you have everyone turn your back on you. They're with during the good and against you during the bad.