If you found out that despite people rallying for ends to injustice and inequality; that there is a type of inequality people support, persecute based on, and revel in - then that's quite mentally crippling. Especially if you are part of the group that has experienced injustice based on it.
People are usually only blackpilled because they look for reasons as to why their life has been so shit. The blackpill solidifies that not only has their life been shit because of something they can't control, but that any attempt to make life not-shit will fail because of cemented societal doctrines they can't change, and will always remain constant. It's a systematically-endorsed discrimination. It's hard not to feel nihilistic in the face of that.