If it released in 2006 sure, things had changed dramatically by 2012 though (thanks largely to Apple and Google). 

All of Nintendo's casual franchises when you really look at it suffered monumnetal collapses in sales, even those on DS to 3DS ... so it's not even limited to the Wii brand.

The Wii brand and whole mini-game crap just ran it's course. That was never going to be popular forever, just as Justin Bieber is not going to be popular forever. 

Switch is selling big numbers but without benefit of a big casual blockbuster, 1-2 Switch is not selling anything that special, in that sense it's doing something even the Wii or DS couldn't do ... both of those needed casual fads like Brain Training and Wii Sports to get sales really ignited ... Switch is doing equal or better than both at launch without that. 

This is among the main reasons; the console market overall hasn't changed all that much, PS4 and Xbox One are doing more or less the exact same thing the PS3 and 360 did and there's still an audience for it when executed decently (and decent is the classification of the combined sales the 8th gen bros deserve, no more). But one large segment, namely the one that was Nintendo's bread and butter, casuals, fringe consumers and entertainment nomads seeking perceived value and convenience above all, more or less disappeared from the more static devices and this was clear for all to see (except for Nintendo, it seems).

The funniest part about this development is that the OP of the infamous UNITY thread claimed that the Wii U would murder Sony and MS' gaming divisions due to a video game crash that would result in mobile/browser/social gaming going bust at the same time as traditional consoles, forcing all developers for some magical reason, over to the Wii U where all would be heaven and glory (it really is an immensely terrible line of reasoning). What makes it funny though is that there really was a crash; the above mentioned demographics completely ditched static devices in favor of smart devices or other, simpler and often cheaper solutions, which caused the Wii U's demise, all the while leading to the mobile/browser/social gaming segments growing and growing each year, and they still are, and the traditional 3rd party developers and their traditional games are doing really well as well, especially now that digital has become even more of a factor on consoles, more people are paying for online subscriptions and the Wii U was just terribly optimized for consumers with even the slightest interest in digital copies, online gaming and matchmaking and basically every other modern contraption and feature on consoles.

In the end; the crash was real, but the effect and afflicted portion of the market was the polar opposite of what our old friend in UNITY claimed, and it is absolutely, spectacularly hilarious in every conceiveable way and I still enjoy going back to read some of the insane shit that was written from time to time and laugh and the ridiculous arguments, theatrics and swollen figures and predictions being tossed around like paint under a bridge.

Yes, but Switch is selling just fine without benefit of a casual blockbuster at all, so it seems like even Nintendo has found the formula for success without needing a Wii Sports/Brain Training/Nintendogs to carry them. 

Good marketing, sleek hardware, tangiable hardware convienance, with relatively decent hardware (given reasonable limitations of a mobile chip), and really great, epic launch title work. Go figure.

Because it was a pain to turn the console on, because the gamepad lasted for about 3 hours, it had to be charged via a plug rather than USB and even when the machine was turned off the battery in the gamepad slowly drained. Come back to it after a week or two and the things flat.

Everytime I thought of playing the WiiU I realised the gamepad battery would be completely dead and need plugging in.

Not only that but because of the tiny storage it also needed an external hdd. So that's three plug sockets. It's just a colossal pain in the ass.

Well mayb ethis isn't why the WiiU failed, but it's why I hate it.

Consoles win or lose based on image. What label a console gets out of the gates almost always dooms or propels it down the line. Wii U was confusing, niche and childish when it launched. It shared a name of a console (Wii) that was not enjoyed by "core" gamers. Once it got a bad label, that was it. Consoles do not storm back from that sort of thing no matter how good the library of games is.

Yeah GameCube largely got stuck with the same rap too -- it looked like a Fisher Price toy so it got labelled "kiddie console" right from the get go and no amount of Resident Evil was going to change that. It's actually remarkable how much a console's ultimate success relies on the marketing/messaging/perception delivered in the first couple of months of its life. 

There's very, very few systems that have ever really "changed their image" over time, you are right about that. I think Sega Genesis was maybe one, but I can't think of many others. 

I like the off-tv concept.
But the rest was pretty obvious..
Bad choice of name... and the bad marketing confused consumers what it actually was. They thought that it was yet another addition to the Wii like Wii Fit.

Bad performance. Not that the PS4 and Xbox One were on the way, the System itself was pretty slow in the beginning. It was fixed later on, but the damage was already done.

Thus the incoming of the more powerful systems the major 3rd party developers dropped the Wii u support, because it was a chore of implementing those engine to a slower device. This and the lack of a huge installbase made it also risky to develop for it.

Nintendo also didn't really knew how to successfully implement the gamepad into the game where it's not being a complete pain (Starfox Zero)

Some first party games had to be postponed due to difficulties in development for HD. So they said...

The problem was Nintendo releasing a home console despite nobody wanting a Nintendo home console ever again. At least they learned from their mistake.

They failed because the market was saturated on the existing console generation at that time, and hugely because the Wii U was named Wii U. It was to close a naming til the Wii. Just the core gamers understood the marketing deal and the console differences.
If named differently, the Wii U would have sold much more.
Lets face it, the concept of the console was unique and attractive, but had some flaws.
The Switch is what the Wii U should have been.
Third party support vanished after they realized that consumers were not interested in the Wii U and because sales were poor.

Price, power, name, and generally failing to capture people's interest. The core audience didn't care about it because of its weak performance-price ratio, and it didn't appeal to more casual gamers because of its confusing name and because it simply didn't have the cool factor Wii had (thanks to its control scheme and marketing, mostly). It was basically an underpowered home console with a ridiculously expensive controller with a largely useless screen in it. If I had to guess, it didn't seem cool to casual gamers, and I can't blame them. The controller is almost the definition of 'not cool'.