Anal sex in porn started in the US market. The US is the largest consumer market for European porn. Quid est demonstrandum.
It's just a taboo, that's why people do it. I could mention that it's in particular a California thing, mentioning the US reliance on horses and all that entails, and the gold rush to San Francisco creating a 90%+ male community that bred an awful amount of tolerance for man-on-man sex, leading to the society that SF now has, but that's just a theory.