Because they are horrible compared to Nintendo games.

"If you don't count blondes, redheads, brunettes, everyone in the world has brown hair!"

Third party games that sold to more than 10% of their installed base since The Wii U launched:
GTA 5
COD BOP2
COD Ghosts
and almost FIFA 13

Wii U games that sold to more than 10% of their installed base since The Wii U launched:
NSMBU and Luigi U
SM3DW
Wii Party U
LoZ WW
Pikmin 3
Lego City
Zombi U
Monster Hunter
DK TF

Notable games that have sold more than 2 millions the Wii U have missed out on:
GTA
Minecraft
Battlefield
Far Cry
Tomb Raider
Hitman
Bioshock
Saints Row
Diablo

So it seems with the inclusion of MK8 and Smash Bros many Wii U owners will already own about 10 games. As a Tie Ratio of 10 is considered good, it seems that Wii U owners have enough games to play already.
The rich among us will always want more and it would be nice if Nintendo could provide it to us, but this is where they have to improve.

It is confirmed?

I thought it would the the inverse:

As wii U W_D will be delayed, they will put all DLCs included in the game.

I do

But I'm guessing the average Nintendo owner doesn't buy them for one of the reasons:

- isn't interested
- doesn't have the time
- doesn't have the money
- prefers to play them on another system
- holds a grudge against the publishing companies for some reason

*shrug*

Only according to Nintendo fans. According to me they are way superior compared to all Nintendo games except Pokemon.

All numbers according to VGChartz database:
Wii U has sold 6.1 million consoles and 23 million retail software. (Ratio = 3.7)
PS 4 has sold 7.2 million consoles and 16.3 million retail software. (Ratio = 2.3)
Xone has sold 4.4 million consoles and 12.5 million retail software. (Ratio = 2.8)

With one year head start Wii U owners buy more games but as it seems they prefer first-party.

Of course we do, that's where the best games come from.

Or more 1st party games have been getting heavily bundled in comparison to PS4 and XBO. 

Corrected.