Maybe true about handheld hybrid boosting sales more than motion controls, but you are stretching it if you say it's a strong library of games...  It's improving, but far from what I would consider strong (yet).  I consider it far worse than the Wii U at the same point in time.

That's essentially the biggest reason. 
Xbox, PlayStation, and Nintendo are all strong brands of hardware and all of those names resonate and are widely recognized. But it's to software that pushes that brand of hardware forward and helps it evolve. 

That's why all of Nintendo's handhelds have been very popular and consistent money makers because no other hardware out there has Mario and Pokemon in its library (mainline, anyways).

That's why Sony was able to turn the ship around when the PS3 got off to a slow start and eventually overtake the 360, because of games and franchises like Uncharted, Gran Turismo, Last of Us, God of War, LittleBigPlanet, etc. that have become synonymous with not just PlayStation but with gaming. Those franchises were able to turn the tide in the PS3's favor and it gathered momentum that just kept on building and building to where Sony is at now with PS4. 

Meanwhile, you look at Microsoft, they don't have any extensive library of exclusive games that can push the brand forward or that is synonymous with Xbox and gaming. Certainly nowhere near the level of Sony or Nintendo, at the very least. That's why they're currently dead last this year in their biggest market, the U.S.. Not only are they not the #1 system in their own home market, they're not even the #2 system.

Halo, Gears of War and Forza aren't going to cut. Not when Nintendo is expanding and making more franchises and not just sticking to Mario, Pokemon, and Zelda; Not when Sony is expanding and making more franchises and not just sticking to Uncharted, Gran Turismo, and God of War. 

for a nintendo console it sure is. when has nintedo ever released so many heavy hitters on year, with this much critical acclaim, probably never. 

Woah hold on. You’re saying the Wii U same time frame of 9 months had better games than the switch? Everyone knows after launch there was practically nothing to play on Wii U. Switch already has Mario Odyssey, Zelda BotW, Mario kart 8 DX, Splatoon 2, arms, Mario rabbids, and a number of third party titles. Wii U only ever got ports Assassin creed 3, Batman Arkham, tekken tag 2, and ninja gaiden 3 at launch. After that most third party games dried up. 

The switch’s library is much better for the same point in time. 

It also had the Wii's library (although some requiring you to also buy a wii controller).  Plus, other Wii U titles you are skipping like  Super Mario Bros U, Nano Assult Neo, Mass Effect 3, Rabbids Land.  Basically, neither was that great, and the switch line up is many times better now than it was at launch.  You are forgetting that many of those you mentioned for the switch were not a launch (but near launch, and I am mentioning some on wii u that were in 4 month launch window).

third party havent sold nintendo consoles since sup nes. nintendo consoles sell because nintendo  games.

I doubt this bro, if this were to be true then MS would have been able to fix it and take the lead back. Instead they continue to screw up all gen with a very poor library and no real direction or brand identity. The XB brand is lost right now, what does the brand represent? 

PS4 is huge because the library followed by the brand recognition outside of NA.

Care to explain the first 3 years of the PS4's sales then? 

Also, mid-gen optional upgrades aren't a new fad. It's just primarily been a handheld thing (new3DS, PSP-2000, Gameboy Advanced SP, etc.)
Is VR a fad? I don't think so. I just think there's tons of room for improvement of that new tech.

1. Microsoft and their crazy policies, even though they reversed most before launch, created an air of negativity around the XO launch.  Most of the insults about its form factor and name and the whole “Xbox go home (you’re drunk)” videos that came out of their conference came because of that negativity

2. Ps4 was $100 cheaper until 6 months into launch for both

3. Sony didn’t make the mistakes they did with ps3.  The fact that ps3 and 360 essentially tied despite the killer combo of $200 price difference and a year difference in launch should have told you how this would turn out.  Granted I didn’t know it would be so lopsided because of point 1 and 2, but I kinda figured Sony would sell more.

4. Sony has had a steady stream of big games from day 1.  Hell they gave resogun for free and it was awesome!

5. Xbox has always been a very focused brand regionally and without that massive US lead the house of cards came crumbling down.  The 360 was essentially everything going right for the Xbox brand and therefore represents its ceiling.  Since they compete for a similar set of consumers Sony had more to sell to

Oh I'm just saying it's one of the reasons why the PS4 is doing so well, there are a ton of other reasons why the machine is doing great, but it does help when your competition comes out the door with a weight around its neck that they are trying to advertise as a feature for their users to enjoy.