Simple, ratings. And idiot studio execs that don't understand that creating a compelling story requires several episodes to provide the background for the show. For example, "The Walking Dead" would have never made it on a broadcast channel because it has lulls where is builds characterization. There was a show like "Harper's Island" that got canceled after 3 (three!) episodes because it was too slow.
On the other hand, this is not that bad of a period of time for horror shows-
American Horror Story
and please someone let me know the others I'm missing, 'cause I know that I am.