Isolation, limited technology, disease, beliefs/religion, and allies were all contributing factors which led to Aztec destruction.
Isolation, due to the fact that the Aztecs thought they were the only people alive apart from the other Indian/Mexican tribes, the Aztecs stayed in their city unaware with the world around them. While the Aztecs were staying in their island, the Spanish were exploring and trading which allowed them to develop better technology such as gunpowder. Futhermore, the Aztecs had no immunity to foreign diseases like smallpox, which killed many of them.
The Aztecs had primitive technology and weapons whereas, the Spanish had an updated technology status and superior weaponry. Therefore,