Because the sci-fi audience is a niche audience. Most "general audiences" will immediately lose interest in a movie if they see an alien or a spaceship or a laser gun. General audiences like things like romantic comedies, action adventures, and so forth.
Sci-fi is getting a bigger audience as time goes on, but it's still pretty small compared to that general audience.
I don't really understand this... sci-fi has never had a problem generating revenue for studio's going all the way back to 2001. Sci-fi on TV is another matter entirely.