The stereotype of Blacks liking fried chicken and watermelon comes from pre-civil-war era southern U.S. where slaves were much more popular than in the north. Fried chicken is a popular traditional food in the south, and many Black slaves grew up around southern homestyle cooking, adopting it into their own culture (and in turn creating "Soul Food", making it even better than it already was). It's akin to asking why Italian-Americans like pizza so much. Sure everyone likes it, but they made it their own.
As for watermelons, melons were a staple crop during the summertime. Freed/escaping slaves that were heading to the north supposedly took a lot of watermelons with them because it's a nutritious food that's easy to transport, and provides you with a shit ton of water when consumed, making it an ideal food to eat while on the run.
So are you saying it's been passed down since the 1800 hundreds?