nintendo's handheld had always been huge sellers, make it capable of outputting home console level graphics and they have a winning formula

It cooks hot cakes (I guess that is what Americans call pancakes, and they sell extremely well in the US).

Because it's the first console in a while to offer a competent alternative to PS/XB without great compromises

The 3DS had many great Nintendo exclusives but it looked like a toy, graphics were disappointing and  lacked Nintendo's multiplayer factor.
The PSVita offered console-like experiences on the go, but lacked of any real killer app and was generally not much supported.
The WiiU had some great Nintendo exclusive but it didn't offer pretty much anything else, plus it was too expensive.

The Switch basically has all of the strenghts of those consoles and none of the downsides.

Yep, Switch is successfully integrating the audience & support of 3DS, Wii U & Vita into a single device.

Hybrid console. The Portable market is huge and N hit it right on the head

Switch is selling well because they shat on everyone who bought a Wii U, and ported over Wii U games so people who didn't buy the Wii U could play them.

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-Super_Boom

Just because you feel that way doesn't mean everyone else will agree with you.

Isn't that a good thing since the Wii U didn't sell very well (bolded part to be specific)?

It's a very bad thing because it destroys the legacy of the Wii U, and Nintendo's legacy as a whole. Now the Wii U will be remembered as a piece of shit, and Nintendo's legacy will be tainted by the Wii U.

It's a portable Wii U that happened to receive Zelda, 3D Mario, Mario Kart and Splatoon in its first year, while it took Wii U 5 years to get those 4 games.

The only legacy the Wii U has is being Nintendo's biggest failure other than the Virtual Boy.