I'm not American. It doesn't make sense to me.
so this question was asked 2 years ago
and here is the one very good on topic comment in regards to watermelons.
I don't have to much to add but i remembered reading an article in "the atlantic" on this very issue, found it on google and saw an interview on NPR by a historian of southern folkways as a source.
She seems to backup the askhistorians comment and shows how the later fried chicken stereotype came about.
The stereotype came about because of the film birth of a nation in a depiction of shiftless black politicians
D.W. Griffith's seminal and supremely racist 1915 silent movie about the supposedly heroic founding of the Ku Klux Klan was a huge sensation when it debuted. One scene in the three-hor features a group of actors portraying shiftless black elected officials acting rowdy and crudely in a legislative hall. (The message to the audience: These are the dangers of letting blacks vote.) Some of the legislators are shown drinking. Others had their feet kicked up on their desks. And one of them was very ostentatiously eating fried chicken.
the fact these are both "finger foods" (i.e. people eating them by default are not using table manners channeling more "primitive" groups and ages) figures prominently.
As a white southerner I regularly eat the stereotyped foods as part of my regular diet. I've always thought the stereotype came from the Great Migration. African Americans moved north so "soul food" became associated with them to a negative degree.