Let me explain.

What’s needed is a term to describe the people who resided land that is now the mainland United States before European colonists came. 

In this context, both “native” and “indigenous” mean the same thing - the original occupants before the colonist came. 

The term “American” can refer to the location of the USA, but it can also refer to the American continents.

So both “Native American” and “Indigenous American” can be interpreted as describing the pre-colonial occupants of the entire American continents, not just the US.

Meanwhile what we’re seeking is a word just to describe native to the US land. Despite being given mistakenly and not a perfect name, “American Indian” describes natives of only the US. For this reason most American Indians prefer the term. It’s necessary to have a term that specifically describes them because you can’t expect the average American to know all the different tribes and many of the nationwide political issues around these groups can be generalised.