Why do american textbooks gloss over events between native american and white americans in the 19th and 20th centuries?

Because it was basically a bloodbath of sickness and violence inflicted on natives by the Europeans.

American History textbooks trivialize the 19th and 20th centuries between the White Americans and Native Americans because (well to start with Native Americans are the most lied-about subset of population...) the time period was a blood bath. The relationships between the natives and Europeans were violence filled and disease.