Mario Kart will always sell well regardless of what system it releases on.

Could say the same for PS4.

I agree with this.

...?  Yes it does.  It has a whole lot of games.

It has access to the entire DS library.  That's already more than enough, but of course for some people that isn't enough.  (But think about it, not all consoles are backwards compatible)

But even not including the DS library, we have a lot of games for the 3DS.

Kid Icarus: Uprising
Luigi's Mansion: Dark Moon
Monster Hunter 3 Ultimate
Monster Hunter 4 (Japan)
Dragon Quest VII (Japan)
Other Dragon Quest games (Japan)
Super Mario 3D Land
Project x Zone (mixed reviews)
Rune Factory 4
Super Street Fighter IV
Dead or Alive: Dimensions
Paper Mario: Sticker Star (mixed reviews)
Kingdom Hearts 3D
Plenty of Harvest Moon games (upcoming and already out)
Animal Crossing: New Leaf
Shin Megami Tensei IV
Shin Megami Tensei: Soul Hackers
Shin Megami Tensei: Devil Survivor (and 2 in Japan) 
Persona Q (Japan)
Etrian Odyssey IV
Etrian Odyssey Untold
Ace Attorney: Dual Destinies
Mario Kart 7
Resident Evil: Revelations
Kirby: Triple Deluxe
Fire Emblem: Awakening
Theatrhythm Final Fantasy (+ Curtain Call)
E.X. Troopers (Japan)
Fantasy Life (Japan)
Yokai Watch (Japan)
Donkey Kong Country Returns
The Legend of Zelda: A Link Between Worlds
The Legend of Zelda: Ocarina of Time 3D
Starfox 64 3D
Tales of the Abyss
Mario & Luigi: Dream  Team
Tomodachi Life
Zero Escape: Virtue's Last Reward
Pokemon X/Y
One Piece: Unlimited World RED (Japan)
Upcoming Pokemon Omega Ruby/Alpha Sapphire
Upcoming Codename S.T.E.A.M.
Upcoming Super Smash Bros 4
Upcoming Yokai Watch 2 (Japan)
Upcoming Mighty No. 9
Upcoming Azure Gunvolt Striker (eShop)
Upcoming Final Fantasy Explorers (Japan)
Upcoming Great Ace Attorney game (Japan)

And there's bound to be more that I've missed.

By your justification, Tearaway would have been a huge Vita hit. People don't buy consoles then find games. They find games and buy consoles.

All console owners are starved for games at the Mario Kart 8 level.

#1, that isn't a full list of all the games the 3DS has, but a list of pretty decent to great games the 3DS has.  #2 Many of the (japan) games are being localized and will be coming soon, and some of the others have the potential to be.  #3 It is not given that not all systems are backwards compatible.  What it means is that it is capable of playing not just its own library, but another systems library as well.  There is nothing wrong with putting that on there.  #4  I also didn't include many digital titles that could easily make the list.  #5 the list I gave by itself for what's already out and available in North America is about 11 games good to great quality games per year the system has been out.  That does NOT include quite a large number of other games that are also pretty good.  

So, actually, there are quite a lot of games on the 3DS.  

I just gave you so many games which released between February and WD on PS4, while only 2 released on Wii U between Feb and MK8. Its obvious which game sold due to quality and which one did due to lack of choice for the consumers.

WD being a 5/10 is your opinion, not everybody's (though something tells me you've not even played the game and are just plain hating on it because it outsold you precious MK8). For me, who actually has played it (though I've yet to finish it) its easily a 9/10. The only other games on PS4 which are better than WD is Infamous: Second Son, Tomb Raider and Battlefield 4.

Yet PS4 had 7 games between February and WD and Wii U had only 2 games between Feb and MK8. That is the whole point. If PS4 owners are zstarved for games with 7 games releasing over 3 months and Wii U owners are not starved despite only 2 games releasing in the same time, then it would prove that Wii U owners are casual gamers, or like you are being hypocrites.

Just look at the history of Mario Kart Series sales and you would clearly see the answer.