Past Ebola incubation periods were under 3 days. This one can be dormant up to 3 weeks. That means you can be a carrier and not know until 3 weeks later. In a place where the health care system is top notch and any outbreak can be contained in a relatively short time, that doesn't mean much. But combine that with crappy health care and ignorant masses, you've got a perfect storm where people who have it don't know they have it or don't want to get treated and thus get other people infected, who then travel somewhere else before showing symptoms and getting other people infected.
This is why it's not as big a deal in the U.S. if it gets here. The people who show signs are quickly quarrantined. The people who are close to them are quarrantined. They'll quarrantine entire towns if necessary.
The only issue is if it hits a big city, and people aren't aware of their symptoms, and it starts spreading. But it's hard to not be aware of your symptoms when you're bleeding out of every orfice. And we do have experimental treatments, worst case. They've already been shown to work. We just don't know if they won't cause worse things to happen in the edge cases, like massive blood clots for certain people or some such.