there is genuinely no reason not to have it
There were no reasons stated, as far as I know.
So it is all guesswork.
The simple reasons are of course, it costs money and will only benefit very few players. Steam has negative filtering and a very limited tag number. I never felt the need to use it there, apart from analytical experiments.
There are over a million games on Itch and negative filtering will not reduce that number much. Filtering away 80k horror games from 1.45 million will still give 1.37 million games. Filtering positive will do that much better. Do not want visual novels? Look for what you want. If you like platformer games, use that tag. There are not many platformer visual novel. Oh, and there are still 500 of those. Why did they tag visual novel? If it is a platformer game, it is not a visual novel. Just because it tells a story, does not make it a vn.
So maybe the main reason is, that tags are not accurate to begin with, to use them for negative filtering. Sure you could filter tags, and a single tag exclusion was implemented quite quickly. 8 years ago. Itch has 8 years of user statistics how and if people would use such a feature.
It will not help you find games you like. But it would hinder you to find games you might have liked.
I picked a game rather randomly from the list of platformer visual novels.
https://intoadream.itch.io/into-a-dream
Tell me, would you consider this a visual novel? It would have disappeared from your browsing, if you would use a negative visual novel filter.