That's because of resources being spread more thin.
if that is the case, and it's at 70% death rate due to the CURRENT strain of 4,000 people having it...
It only makes sense that the 70% rate will RISE

As more people catch the ebola.
Also, I'd guess it's not exactly easy to just bring in more doctors/nurses. They're not an easily renewable resource. And when the first 20 or 100 or 500 doctors catch it, I'm betting it becomes way more difficult to recruit more.
I really could envision a scenario where (as I predicted a month ago) the US military shows up, puts up some road blocks, and just gives a huge part of that region 6 months to heal, die, recover, whatever, but you're not coming out or you are getting shot. And it won't be on CNN lol, it'll just happen and we'll probably never hear about it. I could totally see that being the case. once the doctors abandon the hospitals, once people stop picking up the diseased bodies, it can get ugly fast.
At some point, you can't cure it, you can only hope to contain it.