Many scholars have asked this question, but the question alone isn't well phrased. Many other factors contributed to the destruction of a well-developed civilization whose cities were far greater than those in 16th Century Europe. The Aztec Empire outnumbered the Spanish greatly, but somehow they were defeated. Were the Spanish troops far more equipped with deadlier weapons or did the Spanish hold a secret weapon that destroyed the empire?.
The Aztec religious belief dominated the lives of the Aztec people and ultimately lead to the demise of their civilization. The Aztec's relied on omens and visions to show them the future. In one vision by the aunt of the Aztec's spokesperson, Montezuma, she foretold the destruction of the great city of Tenochtitlan. "She spoke of men clothed in black stone and riding upon hornless deer. They came to Anahuac, and then marched, burning and hunting people down, before coming to Tenochtitlan itself. There they were gathered with Montezuma, and she had seen him lying dead among them. She saw the whole city in flames, while the leader of the horrible, white-faced creatures sat enthroned among the ruins of the city" (Burland and Forman, The Aztecs Gods and Fate in Ancient Mexico, p.112). Visions played a major role in the religious beliefs of the Aztecs. Upon hearing his aunt's vision Montezuma went into a great depression and at times would not speak to anyone. As other visions arose from around the Empire of the Spanish conquers, Montezuma thought that it was time for Quetzalcoatl, an Aztec god of learning and civilization, to return to the throne and destroy the people of Tezcatlipoca in search for his treasure. When Quetzalcoatl returns, he would be a white man and have a long black beard. Montezuma suspected that the Spanish conquistadors was Quetzalcoatl when he heard that they had long, thick beards and wore clothing with many colors.