Question Do you know why Paula quit American Idol?
Answer It seems that the most accurate explanation is that Paula was unhappy with various aspects of how she had been treated over the years by Idol and Fox.
A lot of the talk out there will center on stating that Paula wanted more money and Fox wasn't agreeing to those demands (seeming to imply that it was just she wanted more money with no real explanation).
It does appear to be true that Paula, via her manager (Paula also changed managers in June I believe, likely due to difficulties in the negotiations but I am not certain on that), was requesting increased monetary compensation, but the reasoning for this request was primarily based on how she felt that she had lost out on some endorsement deals specifically due to how the rest of the Idol judges and Fox themselves never sufficiently supported Paula in crushing the rumors about her unreliability and/or substance abuse etc.
So the most simple, and likely 'official', reason would be that Paula and Fox couldn't agree on a new contract.