Why were the Spanish Conquistadors successful in defeating the much more numerous and powerful Aztec Empire?

There were a few factors. The conquistadors had horses, firearms, and diseases. Using horses provided better mobility in battle while firearms greatly outmatched the Aztecs weapons of choice (bows and arrows, spears etc.). Diseases brought over from Europe killed by the hundreds and thousands since the Aztecs had no resistance to them. The conquistadors also had metal armor while the Aztecs only had wood and other less sturdy material to use. The last thing to mention was that the Aztecs had many enemies due to their use of human sacrifice. Many native tribes joined the Spaniards to contribute to their downfall.