Did not quit South Park, Scientology wrote the letter in his name!

https://en.wikipedia.org/wiki/Isaac_Hayes#Departure_from_South_Park

Departure from South Park

On March 13, 2006, a statement was issued in Hayes's name, indicating that he was asking to be released from his contract with Comedy Central, calling recent episodes that satirized religious beliefs intolerant. "There is a place in this world for satire, but there is a time when satire ends and intolerance and bigotry towards religious beliefs of others begins", he was quoted as saying in the press-statement. However, the statement did not directly mention Scientology. A response from Stone said that Hayes's complaints stemmed from the show's criticism of Scientology and that he "has no problem –and he's cashed plenty of checks– with our show making fun of Christians, Muslims, Mormons, or Jews."[28][29]

On March 20, 2006, two days before the episode "The Return of Chef" aired, Roger Friedman of Fox News reported having been told that the March 13 statement was made in Hayes's name, but not by Hayes himself. He wrote: "Isaac Hayes did not quit South Park. My sources say that someone quit it for him. ... Friends in Memphis tell me that Hayes did not issue any statements on his own about South Park. They are mystified."[30] In a 2016 oral history of South Park in The Hollywood Reporter, Hayes's son Isaac Hayes III said the decision to leave the show was made by his father's entourage, all of whom were ardent Scientologists, and that it was made after Hayes suffered a stroke, leaving him vulnerable to outside influence and unable to make such decisions on his own.[31]

The first South Park episode that premiered after Hayes's death, "The China Probrem," was dedicated to him.[32]