There is also the international perspective that says that the US has had a lot of Olympics in the recent past (80,84,96,02) and it is time for the rest of the world to get in the action a little bit more. And the last summer Olympics held in the US wasn't so highly thought of - the head of the IOC implied that it was the worst Olympics ever (Samaranch traditionally said at the closing ceremonies that "This was the best Olympics ever". That statement was noticably absent at Atlanta.)
Time zone isn't an issue with Rio since it is just one hour ahead of Eastern time.