"Space is big" is the main problem. For space rp to work, you really need a GM directing things and keeping everyone together. We thought with the last one we could confine everyone to one system to help things along, but it still had the problem of once you're on spaceship x or planet y, you can't interact with people on spaceship a or planet b. It's a lot easier when you can wander out of Trog's and be at Watchtower as soon as the 60 second post buffer allows you to.
That, and it's just really hard to start new freeform universes, even if there's a lot of support from the beginning.