The Thrilla in Manila was held in the Phillipines because it was a good distraction for the president, Marcos. They were under martial law at the time.

Zaire also had a dictator at the time Mobutu Sese Seko.

Both dictators would have paid a lot of money for the fights.

On the technical side you had satellite transmission starting -- the Thrilla in Manilla was the world's first. This translated into the fights being a TV phenomenon where you didn't need to get so hung up on being conveniently located enough for fans to attend.

The TV dominated to the point that the bouts were at really odd local times to play to US TV audiences.