Its much easier for nintendo to save a flagging handheld with nearly no direct competition than a console with very very strong competition.
But I thought smartphones were very much direct competition, at least they were the most common reason given for the 3DS slump out of the gates? Hell, it's not even just phones any more, 3DS and Vita have to fight off everything from MP3 players and e-readers to tablets as well, pretty much any portable device with an online store which is virtually all of them. Considering that market didn't even exist when the PSP and DS launched, it's no surprise 3DS has struggled to match it's predecessor despite an awesome games line-up compared to the first 30-odd months on DS. I'm surprised its done as well as it has.
I agree a 3DS turnaround was much easier than the job Nintendo is facing now, even taking all of that into account. Their handheld business has been rock solid for 24 years, people have faith in it and the exclusive games that are designed specifically for it like Pokemon, animal crossing etc etc.
WiiU has simultaneously lost the casual market that shaping a controller like a remote with three buttons gained, while also hasn't had seven years of cultivating support for HD action games from third parties.
3DS is fairly traditional as far as hardware goes- it looks like a logical upgrade from a DS, the 3D is non-essential, and it's kept a touch screen, stylus, d-pad and gyro. All it needed was games and support from third parties, and it has the bonus of the games being quite quite cheap to make.
WiiU has many different problems in that it needs to prove that the game pad actually adds something, plus the third party support has been a problem on their home consoles for a while, even for the wildly successful Wii. Plus the new consoles have settled on broadly similar architecture, making it the least likely machine to get multiplatform games. 3DS was pretty much guaranteed to get a shitload of support as a Nintendo handheld, WiiU, not so much.
To Nintendo's credit there isn't a developer in the world that puts out as ridiculously prolific an amount of critically and commercially successful games as them, but they can't prop up the WiiU almost entirely by themselves, no one can, variety is the spice of life. I'm going to give it until Mario Kart releases next spring, and if it still looks bleak then, after what looks like the best line-up they could muster as a rescue plan, I'm not buying into it.