Sci-fi MMOs can succeed very easily.

As long as there is endgame content present (which there wasn't and still isn't in a game like Tabula Rasa, thus there is no reason to actually get to the level cap. Yes, there is a game from lvl 1 and up as well, but if there's nothing to look forward to at the end, people aren't going to bother, simple as that.) and as long as there isn't any huge gamebreaking bugs that prevent people from advancing.

Noone expects a brand new game to immediately match the content of something like WoW, but the devs need to cater to both the hardcore and the casual gamers. Word of mouth spreads like fire in dry grass, and if people aren't happy, the game will likely fail.

Rushing a game and releasing it before it's done = recipe for disaster. Proof? Age of Conan. To a lesser extent, Warhammer Online.

I personally love the sci-fi genre and would love to see an MMO truly push the boundaries and innovate. Problem is that involves risk, which is why a lot of studios opt for a WoW clone instead.