The epidemiology of Marburg and Ebola (infection rates, death rates, incubation etc.) has shown that it is easy to control.
The spread of the virus is easy to stop, but the people who catch it usually die.
I believe that's why it doesn't go world wide. The virus is so deadly it burns itself out fast by quickly killing everyone that gets it. It would be a disaster if a virus like this got into a high population area.