No people for the virus to replicate in or a lack of people i.e. mean distance for the virus to transfer to another person from an infected person. So population density in a critical factor for the scale and speed of its deadly progression.
The virus has come and gone for years in Africa. When "gone" it must be in a reservoir somewhere so it can come back later...
Stopping an outbreak may not be the same as stopping the disease. Are there articles describing where virus has been between outbreaks?