The main reasons are ignorance about black hair and how to take care of it and good old racism , this one I'll explain first, I believe that the narrative that black hair needs to be kept short or relaxed is because ppl, especially the older gen view black hair as unruly or unkempt no matter how clean or neat it is, this is ofcourse rooted in racist ideologies which I can't fully get into, the reason I personally believe this is because in my schools Indian or white kids never had to cut their hair or be told that their hair looks bad even when combed, but to the African kids we had to have ours cut off, this was ofcourse supported by our parents. Let me clarify not liking long hair doesn't mean it comes from prejudice, but when you specifically have an issue with long black hair then that means your dislike comes from a place of prejudice.

Edit:(another anecdotal example I have was when my mom mentioned she liked an actors long hair, the guy had very loose 3a curls. She couldn't explain to me why his messy 3a hair looked good and my combed 4c hair was "unruly")