Beck is a cut-up, which is why his radio show is better than his TV show.
Part of the reason Beck is leaving his TV show is that he feels he can only do so much, in his current format. He's focusing on the internet and a potential comedy show, to reach more youth.
It certainly isn't due to competition from the hacks of MSNBC (even in his down months, his ratings crush MSNBC and CNN combined).