When white people first came to the present day U.S. it caused 90% of the Tribal Native American populations to die. Why?

Because they had no immunity at all to things like Colds and Flu and Bronchitis and smallpox and black Plague and other things that White people Brought from Europe.

The lack of sanitation in European big cities killed millions of white people over the years and sometimes 90% of their populations too. 

However, Native Americans were not really into farming as much as White people of Europe so it wasn't useful to them to get in big cities like White people in Europe did. So, as a result they weren't killed off by the White people's diseases like smallpox and black plague cause usually by infected Rats and vermin more than anything else getting into grain stores harvested from Farms.

So, the white people who survived Europe were immune to these things but native americans were not and this killed (within a few months of contact with white people) 90% of the Native American Tribes contacted over time.

Otherwise White people would not have survived here in the U.S. because the native American populations likely would have killed them all before they were all decimated by white people's diseases.