Mostly, because developing time and resources would have to go on a myriad different things than the things that now make the game great. Balance issues would come out, quests would become suddenly more of a competition, the rp sense of most of the things would be lost as well as many other things.
Just think though, with the idea i had have all the races together on one server. Just 1 of each race not an MMORPG style. The world of skyrim is way to big for 9 races to ruin it. There are all the same armor types so cant really ruin it there. Have one guy be the armor guy and another guy be an enchanter and so on and so forth.



.