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The same reason people bought an N64 or GameCube. Maybe it's not the majority, but there's still a market for people who want Nintendo platforms. Wii was a bit of a different story as it was a break out hit because it brought it lots of casual fans on top of Nintendo fans and was different enough from PS3/360 that even a lot of hardcore players also bought one as a complimentary family/party console.
Wii U is probably more like the N64 or GCN, but that's OK, there's still enough of an audience there for Nintendo to turn a decent profit if they play their cards right from here on out.
I do think though if Nintendo wants more success, they have to shun their conservative nature a little bit now more than ever. Mario and Zelda and DK are great ... but they need to start taking risks with new types of software that has more potential to bring in different users.
Invest more in low budget indie games from the West and get them exclusive for starters, it's a low risk proposition, who knows maybe you stumble upon the next Minecraft or Angry Birds type hit. I would look for more game deals in the vein of the Bayonetta 2 one too. How about a new Contra? Or Megaman?
