I think they may expand offerings down the line at some point, just not right now. If there's money to be made on Wario merchandise eventually (and they find space for items by phasing out stuff that doesn't move), I'm sure they'll give it a try sooner or later.
It's worth remembering that outside of Mario Party and Sports titles, Nintendo internally considers Mario, Yoshi, Donkey Kong, and Wario as distinct properties/IPs/universes that happen to intersect occasionally. They chose to incorporate Mario and Yoshi into the core initial land, with Donkey Kong getting a sub land. Wario exists outside all that with his more modern and cynical WarioWare titles (and the zanier Warioland/Wario World titles, RIP). Birdo is just a niche character to begin with, and I say that as a fan. She also sort of exists on the fringe, being primarily associated with Yoshi but only rarely appearing in Yoshi-fronted titles.