I don't know if you've been paying attention, but Mario is far, far more diluted than Sonic. And Sega failed the transition of Sonic to 3d (which makes zero since considering Sonic Adventure was so good). Nintendo also isn't selling anything gamers want outside of the handheld market. Both the PS4 and Xbox One will eclipse the Wii-U very soon, and how long as that been out?
I don't know, I think the 3DS proves that gamers DO want the Nintendo IPs (and loads of Japanese IPs that show up on there as well, look at all the 1M+, 5M+, 10M+ games on 3DS).
They just don't want an expensive home console with a tablet, and a limited collection of $60 games.
If the WiiU had launched at $199 with SM3DW, no tablet, slightly beefier hardware (or not), it would be doing at least twice as good right now. The tablet thing, high price, and long initial game drought was brutal.
Nintendo is damned smart though, their total profits are vastly higher than Sony/MS. The WiiU weirdly may be profitable before everything is settled this gen. All it needs is to keep building the major IPs. Zelda will be huge, so will Smash and Kart. Doing DK first was kind of derp. SM3DW is a big deal though, if they do a bundle this fall for sub-$200, it's going to start gathering steam as a 2nd-choice console. If they're able to hit $149ish on a bundle it would go bananas. But that stupid tablet thing is probably holding that back quite a bit. The big battery, screen, and wireless radio really add up in cost for not much gain. And for all the use it's had in the real world, it would have been better off being a sold-separately peripheral.
The thing is, someone who already has an XB1 doesn't have a reason to consider a PS4. And vice versa. But someone who has either has plenty of reason to consider a WiiU if the price is right and there's a couple dozen awesome games for it (provided that they like those kinds of games. Not everyone does, but enough people do to make it worthwhile by far).
Mario Kart 8
Zelda New IP
Bayonetta 2
Super Mario WiiU
Super Luigi U
Super Mario 3d World
Pikmin 3
DK Trop Freeze
Child of Light
Hyrule Warriors
Fire Emblem
Super Smash Wii U
Yoshi
Pokemon
Monster Hunter
Metroid (rumored)
Combine that with some more IPs crossing over from 3DS (Bravely Default, Etrian Oddysey, etc) and it should be a solid also-ran.
TLDR : Just read the bolded + decent games + cheaper price.