A lot of things contributed to it. There were rumors to him simply being a prick behind the scenes, that people were complaining about, but ultimately, he simply wasn't worth keeping around. There wasn't an official reason given, but looking at the ridiculous sum he just collected from his contract(that I'm guessing was knocked down a bit), the pathetic ratings he brought in, and the headache he was behind the scenes, I'd say that was the culprit. Don't forget, he was already demoted from his prime time slot before he got fired. The writing was on the wall.
Dude, he was contracted. They can fire him for almost any reason. Not performing up to snuff would be one of those reasons. You don't just get to keep your ten million a year job, just because. They fired him. They paid what they owed him. The end.