I think if 3rd party devs made the kinds of games Nintendo fans actually like, it would be more likely to sell. They need to understand that Nintendo fans aren't like PS and XBox fans. You can't put your dark gritty shooter or your realistic racer on a Nintendo system and expect it to sell. I know we live in a port-dominated market today, but it's just not gonna work for Nintendo systems.

Why do you think Nintendo fans choose Nintendo systems in the first place? Nintendo provides a certain style that you just can't get anywhere else. So if you want to appeal to Nintendo fans, you have to mimic that style. Think color. Think fun yet simple. Emphasize gameplay over story. If Nintendo fans were interested in PS/Xbox-type games, they'd buy a PS/Xbox! Know your audience. Don't bring your Heavy Metal album to a Jazz festival and expect it to sell. Even if it's good.

My 2 cents.

It dates to the beginning. Nintendo had some restrictive policies on NES and to a lesser extent SNES. These had some good impacts and helped the industry rebound and thrive, but some third parties didn't like them. EA in particular felt the platform shouldn't be the most well-known thing, but rather the game developers. That's why for a time back then they would put the lead developer's name in the game title, like a movie star. They said ideally the system would just be a nameless box. But other, more dominant 3rd parties enjoyed their relationship with Nintendo.

Then Nintendo had the dust up with Sony and failed to include a CD ROM drive with N64. This meant that many titles weren't easily doable on N64. Sony swept in and published FF7 which was a major title for Japan and other third parties followed. EA and others were more than happy to move to an environment that lacked Nintendo's first party competition.

Following that, Nintendo made some very questionable hardware decisions like using mini-discs in GCN and low power in Wii and Wii U. These just compounded the problem.

TL;DR:

Two issues:
1. Some companies don't want to compete with Nintendo 1st party (EA)
2. Nintendo alienated the ones who wanted to be on their system by using cartridges on N64.

It should be Nintendo's duty to ensure getting the titles and making a healthy enviroment for them, so, while not necessarily the only one, this issue's biggest culprit is Nintendo itself.

It's a combination of all 3 really, but overall it depends on the game.

Developers:
You can't make crappy ports and expect people to buy them, PERIOD. If you game runs significantly worse, is lacking content, lacking complete modes, and still cost full price why would you expect anyone to buy it.

Nintendo:
When you have developers who push and take advantage of your hardware, give the games the definitive treatment, or even go as far as to make a New / Exclusive IP on your platform, then you need to support the game like you would your own. If you're not support the developers when they develop games for your console why would you expect anyone to want to produce more content.

Fans:
When 3rd party developers make good games for Nintendo, and make games that you would otherwise not be able to ever experience if you're primarily a Nintendo gamer, then do yourself and everyone else a favor and GOOGLE "best 3rd party Nintendo Switch games" and if it's a genre you like buy those games. You are doing nothing but making sure every Nintendo platform is only a first party / Japanese developer device, and Nintendo will bite the dust in a couple generations if that remains the case.

I agree. To further the point with the audience: Just Dance sold a lot of copies - and nearly all of them on Wii, despite the title being multiplat. So there a publisher made a game that catered to the audience of the Wii - and it sold. Let's take some more looks: Monster Hunter sells on 3DS, Ace Attorney sells on DS and 3DS, Yokai Watch sells on 3DS, Hyrule Warriors is one of the best selling Warriors titles since the PS2. So third party sells on Nintendo-platforms - if it keeps the audience in mind. The teenage boys and young adult males that the majority of the game industry is targeting isn't that strong on Nintendo.

Huh? No, no. If Burger King introduced a veggie burger and it didn't sell, would it be the consumers' fault? No. Burger King customers expect a different product altogether, and vegetarians already have better alternatives.

The distributor of veggie burgers might be at fault for placing unrealistic expectations on Burger King consumers, and Burger King might be at fault for not making its restaurants in general more accommodating of special food needs. But the customer is most certainly not at fault.

I've bought plenty of third party games on all Nintendo consoles. I dislike EA, Activision, and Ubisoft regardless of platform. I don't buy their games on either Nintendo or Sony platforms.

It started with Nintendo.

They choose not to use CDs, back when disc space was a issue.

As time went on, and it just became normal that nintendo systems lacked 3rd party, most users adopted a "screw them" mentality, and just support nintendo games. Often a 2nd to own console, where you dont buy 3rd party on it.

Now its no longer Nintendo at fault, its the conditioned consumers too.

The only way out of the hole, would be to do a traditional console, like a PS4 that was on the same level as the then current PS/XB, and not be released 4years or so after them.

Both are to blame, can't just blame one group in my opinion

The number one problem are royalties. As long as the competitors are giving discouts to third parties blockbusters and Nintendo isn't, they're not going to put out the blockbusters.

However, I've never heard anyone saying DS had bad third party support, and neither did Wii, even if it didn't get the blockbusters, so if there is a problem with third parties, it's got to be somewhere else than Nintendo.

Also, Nintendo was the one that made Japanese publishers like Capcom and Konami big in the US market in the 80's and 90's, so I'd say there's a source for dislike, as the US publishers now had a new competition in sight eating their bread.