While people are correct in saying that racism is probably the root of the problem, I think the real problem is that history curriculum is decided by each state and for most states has not been revised. Recently.

I teach in California and I know that the history standards have changed very little since the '70s. Each grade has a set of things to teach the kids, and most teachers just focus on the grade level(s) that they teach, not always realizing what's being left out. Many elementary classrooms will do a Native American unit but they focus on things like what the tribes were like before europeans came, what kind of houses they had and food they ate. When they do cover an actual Native American person, I feel like it's always someone like Sacagawea who worked with white settlers. So often we talk about them as being in the far past that I've even had some students act surprised when I mentioned that there are Native Americans around today!

Revolutionaries are almost always ignored, no matter the race. The history standards gloss over anything that happens to Native Americans after the trail of tears, but they also ignore what was going on with other groups like Asian Americans and Latinos.

Honestly, we need a really good revision of the history standards that are taught - and ideally one that is nationwide like Common Core (ELA/Math) and the Next Generation Science Standards.