I wonder what are the differences between N64/Dreamcast/DS/3DS/PC/iOS and if so, why aren't they single-categories?

The games are very similar, but there is a couple of differences. For example:

N64: You can mess around with the resolution in order to lag the game and clip through some places.

PC: Fast loadings, wrong graps (doesnt work in N64 I think), and atm is the most active.

Dreamcast: Slight differences of the levels, new collectables... I can't tell for sure in terms of speedrunning how that version works.

DS: Port of the N64, but with worse controlls, and some fixes.

3DS: Port of the Dreamcast, but with dropping frames, invisible walls in oob, and you die if you go too far from the level oob, unlike PC. Other fixes as well.

IOS: No clue.

Probably there is more differences, but I didn't play them all. The differences are very small, so people decided at that moment to all being in the same category. But by far the easiest and fastest way to play it is in PC.

What it comes down to is that all versions of this game on this board gameplay-wise are about the same, with some minor differences in the glitches. Versions like the PS1 and PS2-version have been seperated to have their own board as the PS1 removed worlds from the original and most worlds by themselves are completely different and the PS2 version has the overworld mechanic along with more objectives. In terms of speedrunning, at least PS2 can't really be put onto these boards as it has a couple of categories that are exclusive to that version.

Because all versions in this board are very similar and people only really run PC for the most part, there's no point seperating it anyways, especially since N64 has become obsolete after new tricks on PC were discovered to make it faster, though there is a new runner who runs on N64. But versions are still their own thing through filtering. ManoCheese holds the N64 World Record and FreshToast the DreamCast record.