The Spanish Advantage in confronting the Aztec and Inca empires was more than Guns, Germs and Steel* although all of those enabled success.
With a few exceptions the average Conquistador with Cortez was not a veteran of the Italian Wars. The Great Captain was a name to him not a lived experience, he was not drawn from the ranks of Spain’s ever victorious armies.
Often he was a member of the minor gentry living two steps from poverty and desperate for money and status. He was habituated to violence, killing and the pursuit of legal disputes. Lack of status and financial desperation had brought him to the New World in hope of splendid redress. His world view was shaped by the national experience of the Re Conquista. The concept of the long war and overcoming adversity was engraved on his soul. He was an uncompromising if erratic Christian and utterly confident in his civilisation.
The first Spanish battles against the Maya came as a shock to the Conquistadores. Here was an enemy that manoeuvred in formation, that could attack and withdraw at will, that used fire power in a way they recognised. Ultimately the Spanish cavalry saved the day but never the less valuable lessons were learned.
The Mexica had no lasting answer to the horse or Spanish steel, the former always meant native armies could be located and disrupted. The latter that, so long as the Spanish formation held, close fighting casualties would be disproportionately high for their opponents.
Gunpowder weapons and crossbows added to the effect; the gorgeously clad native captains, instantly identifiable by their huge feather banners, could be killed at a distance. Equally a swift advance of horsemen could deliver a lance thrust, a la jinete, to the face leaving the foe leaderless and demoralised by the sudden death of such an able and ferocious captain.
Finally, the Conquistadores had huge numbers of native allies to further tip the balance in their favour.
*Jared Diamond's very interesting book.