It became associated with black culture because black people tended to have larger backyard gatherings, which means feeding a lot of people. They are also historically marginalized, and had lower incomes as a result. So not only were they feeding more people when they had parties; They were doing it for cheaper. Watermelon is a cheap and easy way to feed a dozen people, and fried chicken is cheaper than other forms of protein like steaks. Yes, both are delicious, but the stereotype happened because it was both cheap and could be served in large quantities for larger backyard parties.
I did something similar when my friend moved to another continent. I shipped her a care package (with some stuff she had left behind,) and every single side of the box had some sort of “there’s definitely no SEX TOYS inside of this box” label on it.
When I took it to the post office, the worker laughed and even made sure to avoid covering any of them with the shipping label.