How were the Aztecs so easily defeated by the small group of Spanish Invaders?

The Aztecs weren't as advanced as the conquistadors. The Aztecs had cotton shields and used clubs studded with sharp obsidian rock. Meanwhile, the Spanish had horses the Aztecs had never even seen a horse before in their lives. The Spanish also had gunpowder and metal armor and their guns could easily take out an Aztec jaguar warrior. The Aztecs were also destroyed by the new diseases that where brought over by the Spanish conquistadors. The Aztecs had almost no allies because the other tribes joined the Spanish in taking out the Aztecs.