America has a long history of extremely complex civilisations. They didn't develop in certain areas because they had no need to. When Europeans first landed in (North)America, the technological disparity was noticeable, but hardly dominant. What killed off the Natives wasn't technology, since the technology that would give Europeans an insurmountable edge over the Natives wouldn't exist for a century or so, what killed off the Natives was disease brought by the Europeans.
As for why the Europeans had this technology, that's due to a complex series of factors including but not limited to, geology, biodiversity, geography, and having trade links with Asia and the Middle East that the Americans didn't have access to.
Originally Posted by Honey Badger
I will have to disagree. Just because you figure out how to travel long distances in space means you have a full understand the biology of a completely unknown species.
It's very unlikely that any Earth contains any disease that can be communicated to a species from another planet. It's very unlikely that an alien species could infect any of our species with their diseases either.