Actually I think they are outside of Japan (where the handheld has effectively replaced the home console). We went through a period in the mid 2000s where this was changing with the DS/PSP, but smartphones/tablets have really killed that movement and worse they are now starting to chip away at the kids market too. 

I've never seen a grown adult with a 3DS or Vita in public ever outside of a comic expo, and I constantly fly all over the place. 

It's evident in Nintendo's marketing too, at least in the US, ... all/most 3DS ads are aimed very specifically at kids, it's because their demographic feedback is telling them that that's the main market still buying handhelds. 

Vita is basically the game handheld built specifically for the "older" gamer who wants to play a portable machine ... and it's evident by its sales that the market for that is extremely niche. 

I see adult 3DS owners all the time. Vita's not flopping because it's aimed at adults, it's flopping because (outside Japan) it doesn't have popular IPs like Mario, Pokemon or Zelda to keep the smartphones at bay.

The PSP did well it didn't have Mario or Pokemon, especially in its early days, it was the kinda hot new portable product for a while there. 

I really don't think there are very many grown adults who are willing to carry a completely seperately (and compartively) bulky device with them because they can't go without playing Mario for the 8 hours of the day they are away from the house. 

It's just a very small audience. The handheld market has always been kid-driven, it's just losing that to smartphones/tablets too, we'll have to see how bad the erosion is over the next few years to fully assess how much damage has been done, but right now it doesn't look good. For a while there Nintendo was making some head way with Brain Training and the PSP was popular with a portion of the "dudebros" crowd, but smartphones have effectively killed this. 

One of the problems I think here is that a kid who sees mommy/daddy using a smartphone/tablet all day ... well they want their own. Not neccessarily a 3DS, that's not what mommy/daddy uses, so its not the one they want. I hate to say it but I think you put a tablet and a 3DS in front of a kid, and the majority of today's kids would prefer the tablet. 

The PSP had Monster Hunter to save its bacon, which in Japan is the equivalent of Pokemon or an exclusive GTA. Vita lost MH to 3DS.

PSP was selling very well in North America its early days too. The 3DS routinely actually sells below what the PSP was selling at the same equivalent point in life cycles in North America. 

It would be more correct to say, PsVita is not doing great because most of its potential customers have a PS3 already and they are fine with it, or in other cases they prefer to save money for the ps4.

Same thing could be said for WiiU, most of its potential customers are more attracted by the 3DS (lower price, more games). The only potential selling point of WiiU against the 3DS is local multiplayer and Kart and Smash Bros are not avaliable yet.

That was before smartphones encroached on the market.

How is this not obvious?
@reads thread
Oh.... Wow...
Software sells hardware right gais?
That's why Smash is selling 3DS and not Wii U's right gais?

I'd agree to an extent. The fact that NSMB launched on Wii U and 3DS months apart, as opposed to years apart the previous generation, is something I think hurt Wii U right out of the gate. The similarity of the concepts in 3D World and 3D Land are another harmful factor. Wii U itself is a problem, of course. As a machine, it will never have more than niche appeal. However, failing to ensure any truly new, compelling ideas or software have hit the machine has been a huge failing of Nintendo, and that's exacerbated by them using similar concepts on 3DS for some of their biggest sellers.

This is why nintendo will give 3ds more attention because of better sales

3ds has the leading 3rd party support (not just crappy ports) for any handheld.