Limiting to those decades only because... well, that's when I was in school and I don't know what was taught before or after that.
I'm incredibly fascinated by the cultures that existed in North American before European contact, but I learned absolutely nothing about them in school - in fact I only came to know about them by visiting museums in different parts of the country, and when I first learned about the massive diversity of cultures that existed before European colonization - and the decline of those cultures even before "our" arrival - my mind was blown. It's literally a whole other world that I never even heard of growing up. I was taught European history, post-colonial American history, and a little bit about the Aztecs, but nothing at all about pre-colonial North America. Was that the standard curriculum in American schools, and if so, why wasn't Native American History taught? Is it taught now?
Sympathetically, the nature of history as a discipline was different. Historians and history teachers were interested in the precedents of modern society, and pre-Columbian North Americans aren't a precedent for modern society in North America: their populations and institutions were largely wiped out and replaced by Europeans and their institutions.
Unsympathetically, the historians and historian were mostly white and incurious about the history of non-white people and not especially eager to underline how white people displaced Native Americans: an overwhelming advantage in military power deployed in the utter disregard of what would have been seen (in white people) as sovereign rights and powers.
In both cases, as subaltern studies, anti-colonialism, etc., became more significant in the academy, and minorities became more prominent among academic historians, history teachers in schools, assistant superintendents of curriculum, etc. naturally moved to embrace thinking about pre-Columbian societies and ultimately celebrating them.
It might be helpful if you could specify where you attended school. Different states' cirricula can vary; I remember that my grade school social studies (NY, 1990s) spent a lot of time on the Iroquois, for instance.
(I'm not including that anecdote as an answer, mind you, only as an illustration as to why it's important for OP to add additional information if they want quality answers.)