In the Aztec army, status was derived from taking prisoners. Weapons were designed to wound and debilitate enemies without killing them, because almost every war the Aztecs participated in was a garland war. A large part of why so few Spaniards died was probably the fact that the Aztecs would try to drag them off rather than kill them in the field. The Spaniards had no such problem. They also had steel swords that could punch through Aztec armor and kill with one blow.

And although the effect of guns and horses was very real, the vast majority of Cortez's men were swordsmen. If I remember correctly, there were only about a dozen horses with them at the start of the expedition, and about as many guns and crossbows.