ScrewballHighlander was always a meme format that older players tend to leave after a few seasons of meming and the novelty wears off. The lack of new blood caused by this being an 11 year old game with no dev support and more alternative meme formats means there are less players interested in HL in general. Combine this with the difficulty of organizing 9 players and finding someone who even wants to play one of each class and you have a recipe for a dying format.
Exactly, highlander on payload maps has basically been solved anyways. You have to have your sentry in x spot, have your combo hold y spot, etc. There's no real room for innovation or new strategy like there is in sixes which is much more open. There's a reason why highlander gets boring and most players leave after a certain amount of time.