Honestly? Because you were in remedial classes. If you were in AP history classes you would have been taught for the tests and expected to have a much broader knowledge of history than that.
There is a general expectation with people that are in lower level courses that what they are taught is going to go in one ear and out the other so what is the point in getting too deep into the history of any specific person or event beyond the shallow stuff.
Part of me wishes I could redo high school and try AP history. I think the me of today could have handled it. But not sure about back then