Because you (meaning kids in white schools) didn't explore. Because you didn't mistrust your white and white-passing teachers. Because you let your parents and community and the MSM determine your knowledge about the country you live in (which applies to everyone white and white passing).
I know you personally recognize this later in your article
You talk about repeatedly reading "I have a dream" by MLK Jr. Did it not occur to you that he must have written other stuff, spoken other words? Google exists, right? The second hit would have been "Letter from Birmingham Jail".
Anyway, good for you to be pushing for an ethnic studies curriculum in American HS. In the meantime (10-20 years), go back and lead a BLM rally at your ex-HS, that's what some college freshmen (and rising) did this summer in the South Bay, so younger students (and some parents) can start understanding sooner. But you will get push-back from your parents, collectively.