The reason it has not is that it spreads slowly.
Ebola is highly infectious at certain stages but not, thankfully, highly contagious because it is not easily aerosolized. If it ever mutates to a contagion like influenza it wll be lights out Irene because if we know anything from the few cases brought here it is that our medical communit is grossly unprepared for a highly contagious and infectious virus with mortality rates greater than 50%.