The critical distinction here is that Ebola doesn't seem to infect the outer part of the respiratory system like colds or influenza. There's not a huge mass of cells there to be shedding viri and getting into the air, and it would take more than a few mutations for that to change.

Ebola viri have been found in saliva, tears, can come from bleeding in the respiratory system, so it obviously can get airborne that way. Exactly what that means (how big are the droplets, how long do they stay in the air etc.), how significant that is as a mode of transmission (which includes iffy estimates of how many viri (virus particles) are required to make an infective dose) ... we just don't know. But with our current Ebola importation policy, it's very likely we will start to get answers to these questions....