It is a historical fact that adult content is more popular with men then with women. Therefore, there is little surprise that there are more porn actresses than actors since porn studios have to satisfy the demand of the viewers and also to provide variety and choice.
On the other hand, it is also a well-known fact that for men, it is harder to become a porn actor than for women. While actresses can be of various sizes and forms, to satisfy the demand, men who want to become a sex model should have proper sizes and physique, and meet a list of other requirements from porn studios. Also, men have to do much harder work than women on the sex film set. So, again, there is little surprise that there are more women than men who work as porn models.