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.
Ahem: grape jelly is the default because it’s cheaper to make and has a stronger flavor, therefore you need less of it as an ingredient. Having said that strawberry jam for the win.