Period 1 begins with 1491. If the American Indian population in what is now the United States population was nearly 10 million before 1492, why is the United States population in modern times only 2 to 3% American Indian?

That is because most of them died due to colonization. A huge amount of them died due to germs and diseases that the Colonists brought from Europe without even knowing and against which the Natives didn't have immunity. A huge number also died during their resettlement to reservations. They also died due to wars against the colonists.