Idiomatic combinations of peanut butter, jelly, and bread
The answer is mostly just "it's tradition", as with most questions like this.

I do think the pattern you've described isn't quite the actual one. What really happens is that we tend to eat peanut butter and jelly on sandwiches, and put one or the other on single slices of bread, because with a sandwich you can spread one thing on each half and put them together, but with a single slice of bread it's messy. Sandwiches might be on toasted bread, though - untoasted is common and easier but plenty of people like toasted bread for sandwiches. Similarly, there are certainly lazy people out there who'll just spread something on a slice of untoasted bread.

Beyond that, it's just what we do.