Ebola is surprisingly less contagious than you give it credit for. Measles, for example, is 9x more contagious (measles is one of the most contagious known diseases), and several other common diseases were more contagious. That coupled with the fact that the high mortality rate means that it would probably occur in localized outbreaks and people would eventually figure out that you should leave someone with ebola alone.
This is over time, of course. At first it could wipe out large populatioms in Africa and wherever it spread.