Individual cities bid for the Olympics. Some richer countries have hosted multiple Games, a few cities have even done so (London, Los Angeles, even Lake Placid)
These days, there is much competition, which is why the IOC is working to spread the Games to various countries and continents. A new city will have a better chance (all other factors being equal). For example, if an African country make a reasonable bid, they would be more likely to win that say a US city or some place in Europe.