Micro$oft wont allow them, they don't want mods to take away from their precious DLC sales, not unless they could charge for mods, which the authors would never approve of.

There are many mods which are incompatible with one another, and many which are out of date or discontinued. If we were to cut down the list to those of quality mods which add to the game and are interoperable then we would be imo around the 10k marker. Then If we factor in a developer verifications system we would probably be down to the 1000 marker, just due to mods not passing for things like incorrect scripting code, bad door placements etc. I think the game would have MASSIVE appeal for this feature alone, and thats on top of the "usual" crowd of TES players.