How did Wierwille gain control over his followers?
That's easy. He taught some genuine (though plagiarised) Bible, taught some good, useful, interesting, stuff. And he was able to get away with it because either the victims were badly taught in churches they'd attended; or they'd not attended church before, didn't know much about God or the Bible; and in both cases this new course or class showed them something new. If you'd never heard stuff taught like this before, and been shown Bible verses to back up what was being said, this was something new. If you're hungry, you'll eat anything.
And then, you were undoubtedly surrounded by a bunch of really happy, enthusiastic young people who genuinely appeared to love God, believe God, and had passion in their lives. Who doesn't want to be around enthusiastic and happy people?
So if this is the only place you've heard what seems to be genuine Bible teaching, where you haven't heard things like this before, where you've become disillusioned with the church(es) you've been to previously where you hate stuff that isn't doctrinal, or where practices don't line up with what you perceive of God - why wouldn't you stick around?
Many people here, and others that I've known, have said that they were delivered from the idolatry of Catholicism. But they might (or might not) have ventured into a protestant church of some ilk, and not found that much better. But here - here, was something that seemed to have depth.
For myself, even though I was aware of discrepancies, things that just didn't quite fit together in PFAL, well, I still hadn't heard as much Bible taught as this (initially). I gave the Leecher, sorry, the Teacher, the benefit of the doubt - we all make mistakes, especially when speaking more or less extempore (haha).