It's like people overcorrecting and using "whom" when "who" really would be correct. Ditto "you and I" vs "you and me". People get corrected enough times to be embarrassed, but still don't have any interest in correct usage, so they just blanket apply what they think is the rule rather than trying to actually learn any of its nuances. It's not a perfect analogy, but I can imagine people just reverting to "African-American" as a no-thought safe bet when referring to brown people.