It's a  3ds successor as well as Wii U that happened to receive Zelda, 3D Mario, Mario Kart and Splatoon in its first year, this is the right answer, aslo Japanese games to be in much higher demand. 

Personally I have between 200 to 300 titles on the WiiU.  The Switch does not change that in any way. I can get where you might feel betrayed but for me the WiiU is and will be for some time my Retro machine.  The majority of my games are Virtual Console, Mario's, Zelda's, Metroid's, Mega-Man's, Castilvania's, Tecmo Bowl, Ice Hokey then there are the 40 or so retail games I bought not to mention a host of Indies.  I don't need a TV to play most of them. I know a good many people don't like the WiiU but I was very happy with it (scrolling through page after page of classics doesn't get old for me)

GTA5 being on more than one device does not destroy the game's legacy it only adds to it.  If that game were on but one system do you think it would see the sales it has? Reached the audience it has? I know none of this will likely make you feel less betrayed but not all of us that bought WiiU's are upset.

Combination of the perfect handheld hardware that satisfies majority and one of the best first party years in a while.

The Joycon are clever and allow for each play style to suit different demographics. Visuals from an SoC less than two years old (and one that was a year ahead of most of the competition at launch) allowed for visuals that appeal to most people and showed a suitable leap that justified the price.

The line up this Spring might look like crap to most Wii U owners, but to the majority it looks like a handheld that's still providing one new meaty experience a month to play. Leveraging the Wii U's lineup has been crucial to this success. 

A portable Wii U was what I wanted out of the next Nintendo handheld, and we got that and then some. 

Games and marketing

Guess it ruins BotW's legacy because it was ported to Switch and released alongside the Wii U version. Same with Mario Odyssey since it could've been a Wii U title that eventually was moved to Switch. :P

People tend to forget that while Nintendo's home console performance has been a roller coaster ride, they have completely and consistently dominated the handheld market since the Gameboy days. Switch is just as much a handheld as a home console. In fact - I probably use it 90% of the time in handheld/tabletop mode. So I think that's the biggest factor, along with the convenience convenience of swapping to and from handheld and console mode seamlessly, and the ability to play home console level games on a handheld (even if many of those are home console games circa 2012).

Then there's the sheer quantity of games. I think people often tend to gravitate to the consoles with the most games (PS2, Wii, Switch), especially when you factor in the steady flow of indie releases.

Oh: And finally - it is home to one of the strongest Zelda releases this side of OOT and the original.

Completely dominated seems inaccurate to say the least with the PSP selling over 80 million which is close to ps3 and 360 numbers.  Also with well over a billion smart phones sold a year with a very high percentage of users playing games on them, most portable gaming is on android or ios. I guess Nintendo platforms are a premium niche in portable gaming. You see some android games with 100s of millions of installs on the google playstore. Angry birds etc and you get the same game as both full or free listed on google playstore and sometimes updated different versions plus you have other apk stores like amazon and piracy where people download the full versions of apk without paying.  God knows how many people have actually played angry birds in total for example. I would of thought only android could make any serious claim to dominate portable gaming although that would be casual gaming mostly. However a part of those android installs would be set top boxes and maybe android tv's but surely in the  minority compared to phones and tablets. Just looking at super mario run on the playstore its between 100 million and  1/2 billion installs. 1/2 billion installs  is not something Zelda BOTW will be getting anytime soon and just one of 100s of games in that catagory. Can't even dismiss android as casual only nowadays there are many android games with very impressive graphics engines close or exceeding ps3/360 level.

No it's pretty accurate. psp sold 80 million, but DS sold nearly double the amount, then 3ds vs vita well we know the out come of that.

It's very obvious, great concept, great games and great marketing.

Well yes, PSP was about as close as a competitor has gotten, and maybe it wasn't the near monopoly that Nintendo held for the Gameboy line and basically the 3DS, but the DS still obliterated it (and just about every other console not named PS2 for that matter) in total sales.

I'm talking dedicated handheld gaming devices.

Mobile doesn't really count for my money as phones/tablets are multi-purpose. It's like saying PC gaming dominates the home console market because PCs obviously sell the best.