Richard Preston made the point that Ebola is a failure of a virus: a good pathogen lets its hosts live long enough to spread contagion far and wide. A long incubation is prime. There's also no advantage to lethality - the more of your hosts you kill, the less likely the disease is to spread. Ebola hits like lightning, looks freaky as fuck, and kills you very dead. The 1990 outbreak had a fatality rate of 90%. Smallpox, on the other hand, spreads easily, incapacitates, scars and kills. Humans lived with Smallpox for millenia. It's well-adapted to preying on us. You'd have to be crazy to weaponize Ebola. Smallpox? Been there, done that. Anthrax? Anthrax is the gold standard of area denial bioweapons because it's stable as hell, incubates forever, spreads easily and the only way to kill it is to bake the dirt it lives in to 400 degrees. The holy grail of apocalyptic bioagents is airborne rabies, though. Pull that one off and you've got 28 days later.