There are more arse people than hair people. Hair is displayed while posing and stripping, but in doggy or (R)CG directors prefer butts (and penetration) to be in direct camera view. It turns more people on.

Andreina once commented that she cut(s) her thigh-length hair (which she liked!) to tailbone length because that displays her arse. I'm not sure why she cut back it further after leaving porn. For hair like hers or like Domino's its significant, you'll see much less moving-forward in the case of mid back length.