Why anal?

Serious question. Why is there so much anal sex in porn? There are several sites (probably too many) devoted to strictly anal sex. The woman have gorgeous vaginas, beautiful mouth watering labia and clits, but the male actors almost completely ignore that and only go for the asshole. Personally I'm way more obsessed with pussy than asshole, although I'll rim a beautiful female asshole if it is clean.

I'm not saying anal sex is bad, or even undesirable. I don't understand the preoccupation with it in hetero porn.

Can someone explain this to me? Explain it to me like I'm a third grader.