It is because it is not taught. One should not have to get advanced schooling to know this stuff, but we do. I never knew this stuff in in high school. Until we make it mainstream, we will have a problem.
		
		
	 
The irony of all this is that in a contemporary setting, there are who people are trying to rewrite history and make issues of the past seem less inhumane. Only problem is that these idiots like to focus on slavery, when I'll through them a bone, and say, I don't really care, or even bytch about slavery; that's in the past way out of my control. 
Instead I'm more concerned with, the more "blatant acts" of racism and discrimination which happened a mere 40-50 years ago and have they effect Black Americans today. The notion of simply telling black americans to get over feeling oppressed or more importantly feeling the blowback from prior oppression during the civil rights / black codes era seems idiotic, when numerous people who were against fundamental rights of "black" americans, are still alive today, and have children with whom they shared and instilled their worlds feel in. 
Changing the way people think and react on a subconscious level isn't an overnight fixer upper, but rather a generation shift of acceptance.