The primary reason for the diversity of cultures is the sheer size of the Americas together with the lack of population density which one saw in Europe and other areas. In both areas, the abundance of natural resources and lack of intense density meant that none of these societies developed use of the wheel or of metal tools and weaons. The most marked areas of diversity would be between the native inhabitants of North and South America. North American Indians numbered roughly 4,000,000 just before Columbus landed. They were primarily hunter/gatherer societies (with the exception of the Eastern Woodland Indians who were farmers) with no established cities or complex social stratification.
The Indians of Central and South America numbered over 40,000,000. This increased density led to a greater level of development. Many Indian societies in this area built large cities with complex social structure. Hernan Cortez, upon seeing the Aztec Capital of Technochtitlan compared it to Constantinople, then considered the most prosperous city in the known world.
An excellent work which will explore this topic more fully is 1491 by Charles C. Mann.