Why did the conquistadors with small military forces manage to quickly conquer the Indian states of America?

The local natives were very primitive in terms of military technology. They fought with spears and bows. Of course, they could not withstand the iron-clad conquistadors who had firearms and cannons. The local population suffered huge losses. They were forced to bow before the conquerors so as not to be exterminated. Thus, the Spanish conquerors were able to conquer vast areas in South America.