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.
First off, I actually agree with you that strawberry is better, though for strawberry I usually use jam or preserves. I do like grape jelly fine though.
Second, I’ll use this opportunity to describe the best way to make a PB&J. You need three pieces of bread and a toaster. Toast one and only one piece of bread. PB goes on the untoasted, and J on both sides of the toast, which now goes in the middle. Adds texture without altering the flavor profile or making it even messier like potato chips, and it improves the bread-to-filling ratio. Using PB-only on the untoasted slices also makes for a moisture barrier to keep the J from soaking through.