Why aren't there many alternate history movies? Well, what do most movies have? They have protagonists and a few other main characters. They have a story. This story focuses upon these protagonists.
There are basically two ways to make an alternate history movie. Either make a mockumentary, or focus on a small group of people in one event in the alternate history. Only people who like history, and/or people who like alternate history, are going to watch an alternate history mockumentary. Also, if you focus on a small group of people in one event in an alternate history, then why even make the movie about alternate history? It barely makes a difference whether or not it is in an alternate historical setting, or in a regular historical setting.
For example... which movie will attract more viewers and make more money at the box office:
History: A western epic. The plot revolves around three gunslingers competing to find a fortune in buried Confederate gold amid the violent chaos of gunfights, hangings, Civil War battles and prison camps.
Alternate History: A mockumentary about if the CSA won, presented as if it were a British documentary being broadcast on Confederate network television. Fake commercials targeted at slaveowning families are shown throughout the movie.
Also, if you make an alternate history, there is a pretty significant chance that many people who watch it will actually think some of it is real, simply because a lot of people don't care/know much about history.