there might be a good reason
You do not need a good reason to not implement something.
It is the other way round. You need a good reason to implement something. Wishes of individual Itch users are not enough as a reason. There is a plethora of people that wish that Itch would accept crypto currency for example.
Once you have a good enough reason to think about implementing something you can list the obstacles. Usually time and money and structural problems.
But for this, the initial reason to implement it, should be really good, and not only because some people demand it very loudly. I have yet to see a use case where this feature would be benefitial - for all or many users. And acceptance of crypto currency would be benefitial for a lot of users that have trouble with regular payment services. A tangible benefit. Search with tag exclusion is a vague promise that you find your stuff better. Somehow. Someone needs to explain to me, how this would work. I tried this feature out on Steam and only ever used it for a theoretical comparison and not to find any new games.
tl;dr If there is not a good enough reason to implement it, you only need a weak reason to not implement it. The initial hack leafo made back in the day was dirt cheap and quick. That it never got a user interface or was expaned, but also not removed speaks volumes: It is enough to filter out horror and not so many people actually use it to justify expanding on it and it probably would be not so dirt cheap to fully implement it on this filter cross-section thingy that seems to happen with the urls.
But we shall see. This is community. Maybe someone does have a use case that convinces me. Or someone explains how you can do tag exclusion on all those other sites that are supposed to have it. There is obviously a need to talk about this. And if enough people see merit in that feature and request it with the feature request button, maybe it will be implemented in some way or another.
I would rather have it that they spend time to make the recommendations more prominent and better. There is a reason why I never even knew that Steam has this feature. I did not even know at first where to search by tags on that platform, let alone that you could hide them away.