It's a question of resources.
Northern America, during the Ice Age, was pretty much inhospitable - meaning that most of the ones who stayed behind there didn't have the time or resources to develop technology.
On the other hand, South America was heaven. Warm, with lots of food, the first human settlers there could easily devote their time to contemplation of the world around them.
But I think that a more compelling reason is because the Polynesians arrived in South America first - as did the ancient Egyptians. [ this hasn't been proven yet, but it's probably true ]
As a result, their technology was adopted by the Incans and the Aztecs - probably one of the reasons you find pyramids there ( this is just my conjecture )
Actually, the North American tribes weren't all that backward.