Why is the Prophet Mohammed Sacred in Media?

The question about the sacredness of the Prophet Muhammad in media is a fascinating one.  That is in no way a slam to Muhammad, but rather, it is interesting given the lack of scrutiny that has been given to other things on South Park including God and the Devil.  I don't know the answer, but this New York Times editorial is certainly worth reading.  It questions why the Prophet Muhammad is off limits (and many would argue that he should be) but God or the Devil or even Barbara Streisand and Tom Cruise, are/have been skewered by South Park.