Seriously?!!! Because they HATE their hair.
I distinctly remember a period — back in the late '60s — during which "Black is Beautiful" was a big thing. Black women in the media all wore their hair in a style called an "afro."
There were media stories about how Black people should be proud of their appearance, including their hair.
Seems to me that lasted (at most) ten years, maybe less. Perhaps it was during the disco period (Donna Summer?) in which it went into the memory hole. They got it to stick during the period of high "Black Consciousness," but it wasn't sustainable.