I think it's because porn doesn't really consider women at all, their needs are generally visible.
I believe if the porn aimed at women are more mainstream a lot more would watch.
I personally don't watch porn, I feel like it's sort of a sexual mcdonalds, it just makes me feel bad, and unsatisfied.
Though, mainstream porn doesn't seem to appeal to women, Yaoi, which is basically manga/anime guy on guy porn has a big fanbase. There is lots of it, and it's all pretty much aimed at women, and it's popular.
Theres this game, Enzai, it's kind of like picture book and such, you read it, make a choice. It's really graphic, and not so much vanilla, and it's really popular among women.
To say porn in general doesn't appeal to women would be really incorrect, to say that the mainstream porn that you find on typical pornsites, like redtube, and on dvd don't really appeal to women, i'd agree with that.