Networks won't let them, they are afraid of Islamic terrorists attacks. South Park had an episode years back where they had Mohammad in an episode that was a parody of Super Friends, but it was a group of religious people like Jesus, Moses, Buddha, etc and they had Mohammad in it. It was some time after that episode that the Arabs went nuts on this issue and started attacking people for showing Mohammad. So now South Park isn't allowed to show him anymore.
They just recently did a two part episode about how they can't show him. They had a big black bar over the character in part one, and in part two, after receiving threats, Comedy Central even beeped out his name every time it was spoken.
Comedy Central, along with all the other networks, are scared shitless of the Arab terrorists. Below is an article about how they got scared and censored the show even more. Plus they won't show reruns of this episode or put it online like they normally do.
Comedy Central is in charge of Robot Chicken as well, so I'm sure that is the problem.
http://www.nytimes.com/2010/04/23/arts/television/23park.html