More information about the cell components needed for ebola replication would help address the possibility you raise, but the way to determine route of transmission is to test it directly. It seems to me there MUST be some difference if E. Zaire is airborne for them and not us. Well this is actually a very interesting question. The problem is that it would be very hard to answer with out doing an experiment. The first Exp. would be to see whether cultured lung epithelia from each organism have different abilities to internalize the virus.
Some cells are amazingly refractory to virus.
The next thing to do is to test the ability of the virus to replicate in each of cell lines. Obviously, you can't do experimental infections on humans so you would have to work in vitro. There may very well be differences in monkey lung that effect transmission.