Yeah no kidding... voice work on animated things is known to be GOLDEN (esp with the recent CGI hits) and for the fact that they are so easy and low maintenance.
South Park spoofed Scientology years ago as well so why didn't he quit then if that was the reason. Considering the literally heinous things they've said about nearly every organized religion in America, it would be hyocritical of him to help trash everybody else and then quit when his ox got gored. The episode they did on Scientology was meant to be pretty mean but so was the episode they did on Mormons, Catholic priests and I'm sure evangelics aren't thrilled with Jesus having a public access talk show and being ignored by everybody.