Smash Bros. hardly ever acknowledges Hyrule Warriors in general tbh. I think a large part of that is because Nintendo doesn't even publish those games in Japan, that's up to Koei Tecmo. That's certainly the biggest reason there wasn't any HW stuff in the base game, because pretty much everything else in base game Ultimate pertained to games published by Nintendo in Japan (which includes third party stuff like Rayman, Culdcept, Shovel Knight, etc) or games owned by third party companies with playable reps on the roster (Virtual Fighter, Bomberman, Monster Hunter, etc). Weirdly enough, Shantae is the one and only exception to this.
Of course Age of Calamity got a spirit event, but they were already doing a whole bunch of events based on third party stuff like Cuphead, Kunio, Warframe, No More Heroes, etc.