Because microsoft and sony want to be able to monitor what goes into they're systems. Think about it nvde mods, is that something MS and Sony want to be a part of? Plus it would also be difficult as they would have to section off some of the texture and visual enhancement mods because the console wouldn't be able to cope with it. So for this console generation I think it's a definite no.
They have Saints Row: The Third on consoles, I think the're okay with nudity.