People have already mentioned the scale issues of placing a supposedly giant mountain next to an apparently equally tall French chateau, as well as the importance of park individuality. But it's also noteworthy that was also the first steel roller coaster in the world, a fact that can be quite plainly seen (in the now-primitive design of the mountain's rockwork), heard (it is LOUD) and felt (in your lower back) by anyone who rides it. The attraction is a great fit for a park as rich in history as Disneyland, but I don't think there'd be a huge benefit to bring it to WDW without substantially updating it - which, one could say, is exactly what Disney did with Expedition Everest.
From what I understand, Space Mountain was meant to be WDW's replacement for the Matterhorn.