Samus is life samus is love

In context of the meme, please make this happen to me. ;)

But seriously make more Metroid!

Really? Even Rare couldn't do it. DK64 was one of their weakest N64 games. Sure, that game could last you more than 60 hours on your first try but after about 2 hours, you're basically replaying the same events over and over again. The puzzles are the same. The characters' different moves and weapons serve only to do the same exact thing but on a different designated color or symbol. There is an over-emphasis of collecting things while there is a lesser emphasis on level design. 

DK 64 is an example of what happens when a company starts running out of ideas. That same team made the Donkey Kong Country games, Banjo-Kazooie and was working on Banjo-Tooie at the time also had to work on this game. You figure with five characters, there would be a lot of unique innovations with gameplay and there wasn't. The only purpose of the additional characters was to extend the length of the game. With so many platformers being worked on within a 5 year period, there was definitely some creative burnout and they never got a chance to do better on the GameCube.

So the truth is nobody got a 3D Donkey Kong right, not even Rare. Although, if anyone could do it, I would say the EAD Tokyo team (not sure what they're called now) who made Mario Galaxy or the now-defunct Killer instinct team at Rare who made Conker's Bad Fur Day. 

Wii U has a lot of platformers so that was probably one of biggest reasons it didn't sell well and it wasn't offered as a discounted bundle like Splatoon was.
The real question is why did Retro make a platformer when the Wii U really needed more variety.

If DK had the same promotion as Splatoon, I still don't see it selling more than 2M

..but then again, I didn't expect Splatoon to do as well as it is now..

Could also be because Splatoon was a much better and more interesting game...

Is this a joke? This is a joke.

this

So, marketing?
Well then. If that's the excuse we can confirm now that Nintendo doesn't see DK as a big franchise anymore to at least put a decent budget for marketing purposes behind it.

Wow, the shade thrown at Mario Kart 8 and Smash is real!

OP didn't mention biggest reason why Splatoon moved more console DK, because Splatoon is far more mass appealing game than DK.
Basically it would be similar like saying why Smash Bros. or Mario Kart moved more consoles than DK. People need to understand that DK is just another 2D Wii U game while Splatoon is great new IP that become third huge IP alongside with MK and Smash, and becoming even more popular IP than Smash.