The simple answer is that software does sell hardware.  The Wii U didn't have a lot of software.  This reddit thread suggests the Wii U only had 165 physical games in North America.  That is not a lot.
https://www.reddit.com/r/wiiu/comments/6x9i5b/complete_wii_u_usa_physical_release_list/

The more accurate answer is that people buy based on value.  The Wii U hardware was a poor value.  The most obvious way to increase value is to drop the price, but Nintendo didn't do this too much.  The cheapest price for a Wii U was always $300 in NA.  The other really common way to increase value is to increase the library of games.  This is why people say "software sells hardware".  A bigger and better library actually increases the value of the system. 

Think about it this way.  System A has 5 games you want to play and costs $300.  System B has those same 5 games plus 5 more you want to play and costs $300.  Which system would you rather buy?  Everyone is going to choose system B.  It's because system B is the better value.  You pay the same amount for hardware to get access to more games that you want.

So, in the end, software does sell hardware.  Because the real guiding principle is that people want to buy the better value.  The more games a system has that you want to play, the more perceived value it will have for you.

The Nintendo Switch got most of the Wii U library. But the number of ports barely reaches 15.

So, if there were only 15 games to port from the Wii U over 5 years, that does say a lot about the library of the console.

Software only sells hardware if the hardware is compelling to play the software on.

The Wii increased the value of software by adding motion controls as an interface.

The Wii U took away value from software because the whole idea of having to look at two different screens and having only one controller wasn’t an attractive option.

That's clearly not true. First, where are these 80-90% of gamers that don't know the Wii U was a console? Also, if it confused 80-90% of gamers, then where are the reports of tens to hundreds of millions of Wii U games accidentally bought for the Wii? This didn't happen.

Why are you lying about this?

/Your bullshit

While I realize their target demographic is quite different, I think the Xbox Series S/X will be an interesting example on to what degree this is true or not, given that the name is at least as confusing, if not even more so, than the Wii U. 

This is also (one of the reasons) why I think the Series X will sell very poorly, not even matching the Xbox One (which was a dumb name but at least it was clear what it was). 

Because it looked like a fisher price toy, not gonna lie. 

thank you for this thead! two keywords, marketing and gamepad

when marketing didn't work, word of mouth did not either, the gamepad was just a big unappealing, unnecessary thing, and that's exactly why the games work great on switch without it

had they stick with WiiU remotes and a Pro controller, things would be very different, instead the chose an expensive, unconfortable thing as the main selling point

the gamepad killed the Wii brand, should have only been a peripheral, like the Wii balance board for example

Those good games had huge droughts.
The PS3 and Xbox 360 had better games and better support.
Wii U offered a worse experience in every way that mattered and still cost more (the console AND the games)
It wasn't a leap that people were expecting for an 8th gen console and often was a step back from 6th gen hardware.

I pre-ordered mine and loved it but I had the PlayStation and Xbox to pad things out during the dry spells. It just wasn't a good device despite good games.

You're too harsh on the Gamepad. It marked my beginning in couch gaming, which would later be enhanced with the Switch, and it allowed for an good browsing experience. Of course, why on earth would you use the Wii U browser even back then, but I thought it was cool and used it a lot. Games looked nice on it too, besides a few intense colors which looked weird, like red.

I remember even i was sitting there wondering when they announced it if it was just a new controller for Wii. Messaging was horrendous. And sadly great games doesn't always translate to sales