The Aztecs, Maya and Inca were civilizations on par with the prehistoric civilization of the bronze age in the old world. They had not yet been able to produce iron, but the Maya especially were in the process of producing a written language. Unlike the ancient civilizations of the old world, they were able to accomplish all this with almost no diffusion.
The Spanish were able to conquer them because they had superior weapons, tactics, horses and were able to divide and conquer in the case of the Aztecs and Maya. Additionally, they were the some of the most ruthless bunch of bastards history has ever seen. There is an old quip that the conquistadors went to the new world for gold, glory and God, in that order.
It is somewhat akin to the science fiction scenario where two different being meet and fight in space. The possibility of them being anywhere near equal technologically are astronomically minute, if you'll pardon the pun.