Why? It's a great tech demo to showcase Joy-con's capabilities. It's also selling well since it reached 1m before its first holiday. I bet this game will reach, or even surpass, 3m when all is said and done.
It's a tech demo that costs 50 bucks but should have been a pack in. Not to mention it isn't that fun...or quality..or good. But hey, different strokes. I personally don't think it needs to be advertised. Even shitty games sell well.