Americas had only corn and llamas so the development was slower there

The Americas also had varieties of squash, beans, amaranth, potato and several other crops that could be cultivated, stored and eaten. A lot of those fell out of favor because of colonialism. The lack of animal traction made the european style of farming a lot more labor intensive, but a number of native american communities simply didn't need them.

Outside of Central America and the Andean side of South America, there were no "great powers", but hundreds of small, independent groups. It's a lot easier to deal with a village that cannot call for reinforcements than a city that can. Even then, Europeans weren't stupid and knew that they needed to ally with locals, which was key to their success.

When Europeans came with horses, wearing armor and armed with iron swords and cannons the natives were simply unable to confront them in battle.

Which is why they quickly learned they had to use guerilla tactics. Also, in the denser forests of Central and South Americas, cannons were usually too much of a hassle to haul and would often be more of a psychological effect than anything, since you can't aim for shit when you can barely see 100yd in front of you due to vegetation.