Yeah, if you like 2d platformers you got on WiiU NSMBU, NSMBLU and Rayman Legends. Having Donkey Kong Tropical Freeze is nice but not necessary.

Because it's on Wii U.

Because Nintendo franchises are dying a slow agonizing death. All of their main franchises are beginning to fade into the abyss of irrelevancy with the vangaurd of their own destruction being the wii u

I only read the first page but its obvious no one really answered your question there.

1)The install base of the Wii U is not large enough for a game like Donkey Kong to sell amazingly well.

2) Having third party attracts keeps loyal Nintendo gamers on Nintendo platforms solely, but also attracts other gamers who primarily like third party and exclusives secondary. The chances are higher that they will actually buy an exclusive if they have the console. If Nintendo has no third party this variable goes out the window.

3) Nintendo needs to revitalize marketing for kids with Donkey Kong because it was mostly kids in the 90's who grew up with Donkey Kong as protagonist.

4) Remember that the possible sales legs of this game might exist because more people will purchase Wii U's down the road. Nintendo doesn't sell Blockbusters. They sell games, so let the sales come.

5) Nintendo needs to sell the DS gamers on the Wii U because that might be Nintendo's saving grace.

I haven't played either Tropical Freeze or Mario Kart 8. But hypothetically, lets say its possible to bring both games to 3DS with scaled back visuals while keeping gameplay intact.

It wouldn't make sense to bring MK8 because Nintendo needs that to move Wii U hardware. Also, they have a tradition of releasing one MK per platform.

I don't think it would be a bad idea to port DK:TF to 3DS. However, saving that port for Nintendo's next handheld platform might be a smarter strategy.

Just my opinion of course, but both Legends and SM3DW are better. Tropical Freeze is really good, but just not quite up there for me.

This. People were already saturated before Nintendo announced the WiiU. They knew already what type of games are coming to the platform. 

I think it did a decent job for the current situation.
DK just isnt as big as Mario and also not as appealing to casuals.
Considering the small installbase this isnt really anything surprising.
I think it can still sell ~2 Mio. lifetime.

I didnt even finish the firt one yet, so why should i buy the second one so soon? Maybe in 2-3 years.

Obviously, the first reason, which can be applied to all Wii U games, is that the Wii U is selling very poorly and it affects almost all the games on it.

But the main reason, which is specific to this game, is that DK Tropical Freeze isn't the game that people wanted to see coming from Retro Studio. Doing a straight sequel to a 2D platformer was very underwhelming for people who hoped to see something different from Retro Studio. People expected it to become a studio working on bigger and larger projects instead of being confined to 2D platformers, who are liked, but who have been released in great numbers by Nintendo these last years.

People were amazed by DKCR, but the sequel shouldn't have came so quickly.