They probably didn't do cornbread because those biscuits (yes they're called biscuits here) are so delicious that there was no need for cornbread. That would still be nice if they gave you the option though. Man those biscuits with that fake honey and butter they give you slathered all over 'em are so good.
My best bet to why they never did watermelon is because there's not too high of a demand for watermelon at fast food places. Even at some of the more "black" chicken places like Popeye's they don't offer Watermelon.
on a random note, come to think of it I think Israelis actually love watermelon just as much as black people do.