A lot of it is probably because of a lack of software competition with major Nintendo releases. Yes, the Switch has the best third party support of any Nintendo console in quite a while. But I think it says a lot when for example the best selling major third party games on the system only sell about as many units as the most niche major Nintendo products do, somewhere in the Astral Chain - Xenoblade Chronicles 2 tier (and even then, most probably don't make it to the XC2 tier). Whether that's because people only buy Nintendo consoles for Nintendo games or games that benefit from being on the hardware like indie games, or whether that's because third parties haven't made enough of an effort, it's the case.
I would totally buy NSMBUDXD at $30, by the way. It actually was $40 on Amazon recently, and I would have gotten it, but the last half of the year has actually been pretty packed on Switch for me so I'm more than fine waiting. I'd also like the Wii U ports to get a price cut, yes I'm dirty for getting gifted them and for buying one myself (Pikmin 3's demo was just that good), but they probably should be sold at $40 honestly. Especially Tropical Freeze, that was some bullshit. Oh, and Astral Chain should be $40. People should invest in new IP more and keeping it at a high entry fee isn't the best idea, especially when the game has some obvious weakpoints.