I have a guess about why rule 34 (that there is porn about everything) happens. It's easier to be horny and come up with fantasies than it is to be inspired and come up with new ideas.

(Note: I'd like to point out that I'm not implying that porn creators don't have new ideas while non-porn creators usually have new ideas. It's just the trend of slapping franchise characters into standard porn positions I'm talking about)