Well, Family Guy is pretty popular. Not many programs get resurrected from the dead. The funny part about Family Guy is that many of the jokes DON'T have anything to do with the plot. That and they enjoy taking a punchline way, way, way past the point where it should have been ended. The South Park guys are pissy about the fact that somebody else is making people laugh using a different form of comedy. I don't know why jealousy doesn't seem like the most obvious conclusion. If the tables were turned, it certainly about seem like jealousy then.
I don't think it's jealousy at all. I think they were looking for another show that might be likely to show Mohamed and would probably do it in a non-insulting way. Family Guy, with it's constant use of non sequitor humor, is the obvious candidate, and really the only candidate. And the fact that they decided Kyle, who's generally the smartest of the 4 kids, loves the show is something that has to be considered.