Most conquistadors couldn't actually afford armor. That myth comes from later artists' depictions. Guns were also expensive, inaccurate and not good for much else than a big bang on their own.
What really helped the conquistadors (apart from the diseases you already mentioned) were lots of native allies... it wasn't so much that they came in a curbstomped everyone, but that they took advantage of already existing tensions between the Aztecs and other groups.
Most conquistadors actually ended up adopting the natives' tactics because let's face it, it's hot and the Aztecs dressed the way they did for a reason. Plus, it's a lot easier to replace a club, a spear or arrows than to somehow find a new sword if yours breaks or gunpowder/bullets once you run out.