My apologies if the answer should be obvious. I'm brand new to this site and speedrunning in general.

I just got it in my head last night that I could try running Born from a Wish as a way to get into speedrunning since it's so short. I ran through it on normal difficulty about a dozen times, taking my time from about 9:20 down to 6:46. I then decided to look up the leaderboards and saw that for the console versions, regular PC version and the enhanced edition, Born from a Wish only has times submitted under hard difficulty.

Is there some "legitimate" reason for why this is? Is there something about normal difficulty that makes the runs invalid, or is it entirely down to people just generally preferring hard because of whatever amount of increased challenge and keeping the small number of runners in one category?

From what I can tell from my dozen or so runs through normal, there are really only 3 instances where RNG or enemy behavior have the potential to significantly derail the run:

1. The lying figure closest to the stairs at the mansion entrance might be too centered in the path between the stairs and the body, requiring you to push it out of the way instead of sprinting passed.
2. Technically 2 instances, but in the 1F creeper hallway, one can get in your way at the end of your initial pass through right before the stairs, and on the way back through after Blue Creek apartments, all of them can get in the way and bite your ankles.
3. When leaving the Blue Creek apartments, a mannequin can block the doorway/passage between the apartments and the Baldwin mansion.

Otherwise, the entire run is essentially entirely down to optimizing pathing, movement and input.

I've only watched 2 speedruns done on hard, but it seems to be more or less the same, except for the creeper hallway can be potentially deadly, and Maria can also stumble into doors and walls, and over small, traversable ledges.

You forgot about bug room. This is high rng.