There are two major factors that stand out to me. . .
1.When Cortes made his journey to sail from across the ocean many knew the trip would be long and with it taking so long before reaching land, many of his men would not be able to bathe or clean themselves resulting in much of the crew getting sick and sharing it among each other on the boat. Upon landing on the Aztec coast him and his men would bring various diseases that would severely weaken the surrounding native tribes, as disease spread rapidly it would eventually topple the Inca and Aztec empires making them easier to conquer.
2.Cortes was from Spain as we all know and at that point in time Spain had the upper hand when it came to weaponry, Cortes and his men carried musket rifles and crossbows, each of em also wore a metal chest piece as armor. it was sturdy and able to easily brush off the Aztec weapons since they only had primitive weapons such as stone clubs, stone tipped arrows or spears. .Aztecs also did not have body armor like the Spaniards wore which left them at a huge disadvantage in combat as the metal armor could deflect most Aztec weaponry making them even easier to conquer.
hopefully this helps
