Well sure the 3DS is doing well. The handheld market is a totally different beast than the home console market. Nintendo has shown time and time again that they know the handheld market better than anyone else. Everything from the hardware, software, and targeting the right audiences at the right times... Nintendo is on point when it comes to handhelds. Even with competition from smartphones, Nintendo has still managed to match sales of their past generations over their relative timelines. So people are continuing to buy into each new generation of handhelds from Nintendo which is pretty damn impressive if you ask me.
But I think Krisp really hit the nail on the head in regards to the Wii U. Nintendo kind of messed up with the Gamecube (Melee aside of course

) when it came to hardcore gamers. Hardcore Nintendo fans aside, a majority of hardcore gamers moved to other systems. Rather than try to directly compete with the 360 and PS3, they just attacked the market from a completely different angle. The Wii was marketed to a completely different audience, with a completely different gameplay experience. But this move, while an excellent choice at the time, kind of backed them up into a corner.
Nintendo had a choice to make with the next generation. Try to get the casual gamers that they grabbed with the Wii to buy a new system, or go back after the classic gamer audience. Neither of those choices were really good for Nintendo though. Casual gamers don't care about having the newest and greatest thing. They already have a Wii and a few games like Wii Sports, and Wii Fit, and for them that's probably all they need. It fits their gaming needs, and they really don't see the need to spend money on a new system.
They moved away from the hardcore gamers for an entire generation. Sure, there were some excellent titles here and there on the Wii, but the support was lacking relative to the other systems. Nintendo was already loosing this audience during the Gamecube's lifespan, and then basically decided to just let it happen with the Wii. Getting them back would probably be an even bigger uphill battle than convincing casuals that they need a new system.
So the lesser of two evils probably was to just try to keep the casuals. But for that to happen, I think you would need things to be as revolutionary as the Wii was at release. And the Wii U isn't. It doesn't separate itself from the Wii nearly enough.
Hell, I'm a Nintendo fanboy and even I find myself hesitant to get a Wii U. Pikmin is one of my favorite series ever, but one game doesn't make me want drop the cash for a new system. I'm not even that excited for Smash 4, as for me it's all about Melee and Project M. Like, I know that eventually we'll have a new Metroid game rolled out, and a new Legend of Zelda, and the other first party titles we expect... but it's pretty sad that they couldn't bring some of that strength on release. "I'll get it in a year or so" is NOT what you want consumers to be saying.