First is that consoles are very technically limited, Bethesda is going to use every cycle and ever iota of ram they can on the PS3 and 360. Adding even so much as a new sword model could at some point eat up more ram than the consoles have and ruin performance, which is very much a thing that is almost universally expected to remain consistent on consoles. Things like Little Big Planet take user levels into account from the very beginning, The Elderscrolls does not. So point one: It's technically almost impossible to run mods on today's consoles. No, don't argue this, I was once a CS major, and still program on occasion.
But even if it were technically possible, Microsoft and Sony would require heavy moderation of any and all mods available for consoles. The last thing any of them want is parent's calling their help lines complaining that little timmy played a game where he could do cocaine or see someone naked. Then it gets in the paper and Fox News calls Bethesda a smut peddler targeting children and then some moron sends a letter bomb to the Bethesda offices. So Bethesda would have to hire people to look over every last mod, which costs them money and frustration and complaints from people asking why their mod got rejected. Not something they really want to deal with.
So there we go, two reasons why it will never happen on today's consoles. Will next generation consoles have enough ram and cpu power to spare? Possibly. If people want it enough to justify the cost, and if they build the next Elderscroll's to allow this from the very beginning, then yeah we could very well see mods and etc. available then. But there is no way it's going to happen with Skyrim. Unless of course Talos himself appears and commands them. No one argues with a giant dragon god.