The Season 1 finale, which was originally scheduled to be a two-hour episode, has been clipped to 90 minutes. According to show creators It has nothing to do with the popularity of the series or the reception it's received. This is simply a scheduling issue.

Long story short... series creators Ryan Murphy and Brad Falchuk have so much going on they can't find time to cram it all in!
Here's the breakdown per
Deadline: Murphy and Falchuk have
Glee that they also created. So the boys were obligated to complete Season 2 of
Glee before they were able to begin work on the pilot for
American Horror Story, which put them a bit behind the 8-ball as far as scheduling was concerned right off the bat.
The idea of just running a one-hour season finale was discussed, but word is that Ryan Murphy came up with an intriguing 90-minute finale that would fit into the schedule and leave audiences with smiles on their faces, thus satisfying all parties involved. Additionally, fans of the series will be happy to know that the ratings have been very favorable and there has been discussion regarding the second season of
American Horror Story, which has already been picked up, running longer than the originally scheduled 13 episodes.