The top linked answer blames lack of effective copyright enforcement, but I think that the availability of rivalrous homegrown porn is at least as significant. High quality video cameras are now built into millions of phones. The performers don't require scripts, crew, or payment to appear in videos. The product doesn't need localization for global consumption -- French movies are fine for US audiences even without dubbing or subtitles.

This is in contrast to theatrical movies, where Joe Rando's video clips of him and his girlfriend are very rarely an effective substitute for a scripted production with paid actors. Even with hypothetical perfect copyright enforcement on all pornography containing professional actors, the deluge of free amateur content would have dramatically eroded movie pricing by now.