I'm guessing for the same reason that some girls start out on non-nude glamour sites, then do cam show peek-a-boos, then open single girl sites with dildos, then do girl/girl... then full hardcore porn. Stretching out the anticipation = MONEY!
Once they go anal, the next stop is fetish. Think about the anal bang girls who've been around for awhile, and how many of them took that route. Think about how many of them eventually went into fetish: Mika Tan, Lucy Lee, Shy Love, Isis Love, Annie Cruz, Flower Tucci.
With fresh faces coming in everyday, the vets have to find a niche where the new girls won't show up for awhile, I guess.