The switch despite being a slight upgrade from the Wii U has a clear message.

That is in my opinion the main reason. I know several people who did not buy a Wii U since they thought it is a version of the Wii.

Exclusives are the only reason I buy devices. They are the reason I own PS4, XBO, & NSW this gen.

The reason I never bought GCN, Wii, & WiiU was because I didn't see any Exclusives that really hit Home with me. When Super Mario Odyssey was shown, and finally looked like the successor to Super Mario 64, I knew it was only a matter of time before I bought the Switch. The promise of Pokemon made the deal even sweeter. Now that those two games brought me, I am also trying out BOTW, DKTF, MK8D, and SSBU.

Basically Mario and Pokemon have always been the reason I wanted Nintendo devices. Sunshine, Galaxy, NSMB, and every Pokemon after Siver/Gold never really excited me, hence why skipped all Nintendo devices after N64/GBC until Switch (sans the Wii I was gifted, which ended up dieing in before I ever got to seriously invest time in it).

As for PlayStation it is always Gran Turismo that gets me, and later on Killzone. On Xbox Gears is the driver.

I play very few 3rd party titles, and have only enjoy a select few over the years. I could not even imagine buying a devices only for 3rd party titles. Nothing 3rd party has stood the test of time for me. There is no long running 3rd party IP that I consider must have.

First, exclusive games are far more important for Nintendo platform than they are for PS/Xbox consoles, espacily today when 3rd party are more stronger than ever,
look on best selling PS4/XB1 games proves that.

Talking about Wii U, problem with Wii U is that start receiving great and big exclusive system seller game when was already too late and console already considered for fail, first big exclusive game was SM3DW and come out technically at start Wii Us second year, for comparison in some time period Switch already had Zelda BotW, MK8D, Splatoon 2 and Mario Odyssey. If Wii U on launch or its in first year had games like Mario Kart 8, Mario Maker and Splatoon, it would sold better, but its not like games were only Wii Us problem, there was also terrible branding/naming, marketing, appealing, pricing, timing..

While the Wii U had many exclusives, apart from Indies that's almost all they had, thus missing some key multiplats like Fifa (crucial in Europe).

It didn't help that the console had a staggering 53 games at launch (!!!), but most of them were ports of age-old games nobody was interested in anymore, and didn't had much for the months to follow. That botched launch cut the legs of the Wii U right off, with the PS4/XBO announcements practically killing any hype the console had left and the publishers all jumped ship (except Ubisoft)

The marketing was horrible, and the communication at the beginning was even worse. I am pretty sure most parents still think it's just an overpriced tablet controller for the Wii.

Because Nintendo had to support the System by it's own after a couple months, they had to churn out games at a fast pace. As a result the quality of the titles suffered a bit. On the other hand I'm sure this situation also helped Splatoon a lot to get pushed forward.

Many gamers were simply turned off by the Gamepad, thinking it was unwieldy and too heavy

i meant that new super mario bros sorry 
and mario 3D world i forgot oops
tokyo mirage and fatal frame maiden are def not top notch like the other games listed they are simply good but not console sellers.

Isn’t the entire Playstation brand built around exclusives, even if that might not be the case with the PS4 specifically?

The fact that exclusives sell consoles is true, but it is not the only important factor, and if the rest fail, exclusives are going to have an uphill factor keeping the console alive.

The Wii U wasn't portable. That's why.

The biggest problem with the WiiU was the controller. It was ugly, bulky and looked nothing like a standard controller. Just looking at the console makes you not want to play it. WiiU had amazing gems, throughout its short life. Sure, it might have lacked a lot of third party games towards the second half of its life and that greatly contributed to its failure. But it still had a few third party games that really stood out.

However, the number one failing feature the WiiU had was its controller. Its the biggest controller by far that any console has ever had. It looked more like a Childs vtech iPad ripoff than a controller. Finally, it's just not as comfortable as any of Nintendo's previous consolers.

NES - light and smaller
SNES - light and more comfortable
N64 - weird, but extremely comfortable.
GCN - weird but light
Wii - light and comfortable
WiiU - ugly, looks like a childs iPad
Switch, light, extremely comfortable.

The WiiU is Nintendos Virtual Boy, ugly, uncomfortable, and because sales never really took off Nintendo cut its life short.