There are three main reasons for the diversity of cultures in the Americas before European contact. These are the relatively low number of people in the Americas, the varying environments in which they lived, and the difficulty they would have had in communicating with one another.
There were not a huge number of Native Americans populating the Americas before European contact. Scholars do not know how many there were, but the numbers were clearly not large. This meant that there would, in many cases, be large distances between groups of people. This, along with the lack of modern technology, meant that the Native Americans were not in constant contact with one another.
In addition to this, there was the fact that the Native Americans lived in many different environments. There were people living off the sea and people who did a great deal of farming. There were hunter-gatherers who lived in forested areas and those who lived on the plains. These differences would have helped to create different cultures among the Native Americans.
Before European contact, Native Americans were relatively few in number, living in different environments, and not in constant contact in ways that would have shared their cultures. These factors led to a diversity of cultures.