There were good third party games on 3DS, and yes a small few companies increased support. But for every Level 5 or Atlus, there were at least three Sega's or Ubisoft's who never took the console seriously for the most part. 3DS is much closer to the GameCube and Wii in that regard. Okay third party support, but not reliable enough for Nintendo fall back on, thus forcing them to produce much of the big titles themselves. 

*sarcasm* 3DS sucks! It didn't get all those shovelware games from Ubisoft, Rockstar, and EA! That's why I bought a Vita! *sarcasm*

For real though, OP is dissapointed with 3rd party support on the 3DS, because it didn't get CoD? CoD on handhelds has always been trash. If your tastes aren't refined enough to enjoy all the Square-Enix, and Atlus games then don't get a 3DS. 

That´s only true ..... if you just look to Western Developers. Activison (Skylanders) and Warner (Lego and Scribblenauts series) seem to be the few ones who kept supporting it regularly. EA launched a couple of FIFA games too.

On the other side , Japanese developers like Capcom, Atlus, Level 5, Square Enix, Sega, Namco etc. launched many games for the 3ds. Most of them exclusives.

Series like Monster Hunter, Professor Layton, Dragon Quest, Yokai Watch got very good sales, as well as some stand-alone releases, like Fantasy Life, Kingdom Hearts 3D, Megaten games, Etrian Odyssey etc. The system was a paradise for Jrpgs. Overall, it had great 3rd party support. Not exceptional, but great.

Meanwhile, I'm too busy getting hyped for Persona Q2.

I would say that the 3DS has a decent 3rd party library, but it's true that the DS and GBA had even better third party libraries. The biggest problem with the 3DS is that it stalled out of the gate, because it was priced too high, and there wasn't a killer app at launch. I can't blame some third parties looking elsewhere to make their money, and now 7+ years later we see that the 3DS really wasn't nearly as successful as the DS.

In spite of all that there are still a lot of good 3rd party games on the 3DS, especially from Japan. I mean the game library as a whole is a lot better than several of Nintendo's home consoles. But yeah it could have been better.

Even Japan support was lackluster. Capcom just released nothing but Monster Hunter with the occasional AA games after Resident Evil Revelations released. Square Enix just had mostly Dragon Quest remakes and the Bravely games. Atlus and Level 5 were 2 of the only companies who gave the 3DS consistent and varied support, rather than sticking to the same 2-3 franchises. Compared to the DS where Square Enix put out all sorts of games from Final Fantasy, to Mana, to The World Ends With You. 

There were great third party games on the 3DS, but It's hard to say it had great third party support in general since it was so limited in terms of variety. 

The heavy increase in the mobile market through the DS generation had a lot to do with it. Although devs still pushes to have their games on the DS because: A. the DS was still THE platform to make money on for low budget projects through its lifetime. Then the casual market shifted from DS to mobile largely due to the lucrative freemium market. Nintendo, unlike mobile carriers, didn’t allow pinch tactics.

Nintendo didn’t abandon the casual market as they still had Tomodachi Life, the 3DS Streetpass games, Nintendogs and Cats, and Animal Crossing. But the hardware was too weak compared to mobile, and Nintendo’s ethical stance on freemium and pinch tactics that were normal on mobile were too slow to budge.

Although I suppose that’s changed now: Switch has Fortnite which is a shooter freemium game with casual elements (by that, I mean it encourages shorter play sessions, and for users to return frequently for quests and such). Nintendo has also released a casual Fire Emblem on mobile that seems to be profitable.

because the DS western parties migrated to app store. However, I think the 3DS had stellar 3rd party support...Ubisoft, Atlus, Capcom, Namco, NIS, Konami, Activison... a ton of 3rd parties released on it

It was absolute garbage after 2 years for anyone that didn't care about MH or turn based RPGs.

MH and 1P kept me engaged. 4 ultimate was my last 3P purchase. Explorers is the only other late gen 3P action game that comes to mind and I felt satisfied enough with MH as a party action game. 

Those first 2 years were really nice though. A pretty direct continuation of the PSP in many ways. 

I blame the Western support issues on hardware. Mobile GPUs were quickly a gen ahead even if they couldn't produce the same visuals and engines had moved on to support that hardware. Nintendo actively pushed these developers to mobile. Gameloft was a heavy supporter of the DS, but IOS had much better openGL support and was generally a much more exciting platform. This was the period when there was an article every other minute saying mobile was the end of portables (though these articles were definitely more prevalent because of the initial missteps of the 3DS itself).

It was somewhat surprising that even the more prolific Wii 3P supporters didn't attempt to create experiences using those engines as a base. The first year did have a number of (OK to bad) Wii ports to round out the lineup. Then even that was done. 

The paltry software ratio is proof of a weak library. 

Honestly I'm not even sure you can consider the 1st party support for the 3DS that good either. It was ports, remakes, and half-sequels. It says a lot that of all the three main Zelda games included in the library, not a single one was even close to being wholly original. Playing 3DS games on the 3DS is always a fun time, but if you're just looking at the library on paper there is a lot of worthy criticism against it. Thankfully the library is just so big I don't care too much.