I never got that. I mean, Donkey Kong and Diddy Kong got in, complete with awesome remixes of music from Donkey Kong Country games:
And even Luigi's Mansion, Super Mario Galaxy, Super Mario Bros 2 and Yoshi's Woolly World are represented in this title.
So why not Wario? He has an Amiibo. He has an obvious gimmick (shoulder barge through blocks). His levels could have a Wario Land themed tileset and background with a remix of a Shake It theme or something, like this:
Yeah, beats me. The only thing I can think of is that his overalls amiibo from wave 2 of the Super Mario line hasn't been announced/released, but that's a really flimsy excuse if that's the only one.
As far as I can tell from job postings and job interviews I've had, they're working on mobile stuff. Whether that's a new MvsDK for mobile or just like mobile SDK work, who can say. I know they worked on WiiU and Switch firmware/SDK stuff in the past, so it might just be an extension of that work.
They also helped out with Mario Maker 3DS and Snipperclips, so there's that I guess
They've been neglecting Wario in general these days. He doesn't even have a Switch avatar while Donkey Kong does, and his series didn't get a reference in Odyssey while Donkey Kong's and Yoshi's did.