I’m not sure how or why grape became the default. For the longest time, I thought I didn’t like PB&J. But ever since trying it with strawberry instead, I understood the appeal. It’s so much better, and now I find it weird that the combination isn’t more common.
I find marionberry jam to be the superior option along with raspberry, although I agree strawberry is better than grape.
However I have never tried white/gold grape jam (made from green grapes) which I’ve often seen spiced, so I imagine that might have potential.