Nothing, the Wii U just isn't a very appealing product to many. That and there are way more possible ways to game nowadays.

It's not doing "discontinued" bad, if that's what you mean. It's doing "OK".

If you look at those number you can see that the generational difference in home consoles isn't that big actually.

January 27th, 2007: Wii+360+PS3= 14mil consoles

January 26th, 2014: Wii U+X1+PS4= a little less then 14mil consoles

PS3 had just launched

Still, neither 3DS today or PS4 can even dream of close to 400.000 sales weekly.
What I mean is - total weekly sales of January 2014 - roughly 600.000 consoles

Total January 2007 - close to 1 million. 

I don't mean to rub more salt in the wound but the numbers you are linking from January 2007 don't include European sales for PS3, since it hadn't released over here yet.

Its doing Vita bad which is pretty bad as it means it might not even overtake the Vita ever.

True but we have to consider two things here.

1. Of those one million consoles, over 500k were handhelds, which are without a doubt massively down this generation that can't be denied.

2. Another big contributor to the 1 million number was the Wii, which was just a console with a very unique sales curve, freakishly high sales at first but then a relatively fast drop of overall sales as well.

The biggest reason for those high january numbers in 2007 is that Nintendo were able to generate huge sales by tapping into a market that normally wasn't really interested in consoles or handhelds. The 2014 sales on the other hand mostly seem low because Nintendo hasn't been able to generate the same effect with the 3DS and the Wii U (and of course because dedicated handhelds are going away), that is of course bad for Nintendo and dedicated handhelds, but I think console gaming in general will still be doing fine this gen.

It needs that DKC boost. But then, no big games until May with Mario Kart? Hopefully they have something planned between then, even if it's a Luigi's Mansion HD.

I just wondered about the overall decline of sales. I love my Wii U and the only thing that bothers me is wheter it'll get assassins creed 5.

back to the topic - if Wii was an anomaly and this can't be repeated for now, then why my first (home) console was Wii, which got me into gaming and I bought PS3 and Wii U then?

I can't seem to believe that PS4 and 3ds will be selling so well forever. 3ds was perfect in 2013, it's falling a bit now. PS4 might be perfect this year, but it can't go like that forever. A decline in 2015? quite possible IMO.