I would assume it's because the game just came out and further categories are still being explored / discussed before adding them. I think the Discord has tons of discussion about stuff like that, especially since it's where people are mainly directed to when asking questions like these.
Just like Aussie said, categories are not set in stone and game just came out (leaderboards just opened 2 days ago). As for Legacy/No Major Glitches : people are already routing/running it, so I would expect this category to be around here somewhere in the future. ;)
A month ago, it was brought to our attention that the full collection of codexes, which is required for the campaign 100% and 100% Natural Masteries categories, is not possible with our current rules. For example, the extra lives tutorial codex cannot be acquired in the Ultra-Nightmare difficulty. T