I don't know if American Idol is rigged or not (nor do I care). I know the songwriting contest was, and that's probably why it was discontinued.
Either way, that is not the reason why judges leave. When you say "most judges", what do you mean?
The show has run out of steam anyway and interest is waning. Last season was pretty awful. I give it one more season, 2 at the most, although I'm not sure if it can survive another year without Simon.