Shortly: 10 years ago anal was still a specialty compared to pussy fucking...then we discovered that we could have both...with the same lady and for a small investment.
The return instead was bigger for many reasons I do not have to tell you...the simple one: people love it or at least to look at it.
The trend grew until today and I predict it will continue so for the same reasons and also because anal can be chic (forget Max Hardcore...ok?!) and glam or maybe "coquin" how would say Mr Alain Bernardin.
Nothing to add...
Gapes is another argument: the first gapes I saw were disgusting... but the current ones are well shooted: ass is shaved, clean, pink and inviting like a pussy or even more.
More: we know the efforts to record them...especially for the ladies. For me this is a reason more to observe them with admiration and respect.
I know also a lot of sanitary concerns but porn is to make people dreaming...so, sit and enjoy

After all, we are currently enjoying anal and pussy fucking very often altogether....
Who could ask more?