Because it doesn't stack up at competitive level, pretty much everything that it does at least reasonably well has been done and is being done better by other fighters still going competitively. That and the fact it gets patched so often that it makes life harder for anyone trying to take it seriously. The only thing it has in its favour that's head and shoulders above the rest is gore, but I'm a nurse - I see more gore at work, lol.
And I speak from experience - I've finished top 8 in a MK9 tournament myself. (Take all comers, London)
Not a bad fighter to learn how to play FG's on, I will admit, but as a competitive main game? I just don't see the appeal. =/