Hey I don't know if this was asked because I have not seen it, but why are there no categories for other difficulties?

Probably because there aren't enough people running this game. If there was a lot of categories they would be with 1-2 people on the leaderboards (this mistake was already done with P4)

Traditionally SMT and Persona runs used to be done on the highest difficulty. As far as P5 is concerned, due to being able to swap difficulty mid-run, the board initially wasn't only for hard mode but as the runs progressed, pretty much everyone stuck with hard mode only given that it's by far the more interesting run. In addition to that, what PovkaLTU said also applies. There's no point in having dead categories like that on the leaderboards when the amount of people running the game is already pretty small. Of course people can run other difficulties if they want to, but we're trying to not loosely add stuff to the board like it was done with P4 back then.