MBV has a powerful mood feeling - A good example was the use of one of their song at the end of "Lost in Translation" - it felt so great. It definitely does not have the best vocals, and the guitar work is much messier than the rest of the shoegazing scene. However, I feel the music penetrate through the skin rather than the ears when listening to MBV... Not sure if that's the best explanation, but oh well.
Slowdive were the mellower band; they were not as brutal as Ride or (early)-Lush were (read, screechy), and because of that, the music might have withstood the test of time better than the other bands. It's hard to understand nowadays what songs like Seagull or Dreams Burn Down brought to the scene.
Or it's possible that Mojave 3 and Rachel continued to do music, as opposed to all the others bands disappearing, except for Miki who kept being a brat.
By all means, I like all of these bands