First, let's talk about why Nintendo had such good third-party support back in the day, which is actually directly related to why they lost that support so quickly.
"Because Nintendo controlled the production of all cartridges, it was able to enforce strict rules on its third-party developers, which were required to sign a contract by Nintendo that would obligate these parties to develop exclusively for the system, order at least 10,000 cartridges, and only make five games per year.[66] A 1988 shortage of DRAM and ROM chips also reportedly caused Nintendo to only permit 25% of publishers' requests for cartridges. This was an average figure, with some publishers receiving much higher amounts and others almost none.[64] GameSpy noted that Nintendo's "iron-clad terms" made the company many enemies during the 1980s." -- https://en.wikipedia.org/wiki/Nintendo_Entertainment_System
When real, serious competition arrived, it's pretty obvious why the scales tipped. This was a company that treated third-party developers as pawns instead of business partners and had no remorse when their policies bankrupted them.
I think that history needs to be understood when we talk about Nintendo's issues with third-party operators, especially when some people advance the childish notion that third-parties are somehow doing something wrong or improper by not "supporting" Nintendo.
Jumping back to the present, the problem we have now is that Nintendo, for whatever reason, never seemed to learn from the success of its rivals. Sony, in particular, began treating third-party organizations as business partners, which is how it's supposed to be done, and Microsoft came along and extended that philosophy even further.
What I'm saying, in a nutshell, is that the third-party situation with Nintendo isn't going to change unless Nintendo itself changes it. It's all on them to be proactive, it's all on them to make third-parties feel that a Nintendo platform is a good business opportunity, with a good return value on resources invested. That's basic business sense.
I don't know if it's pride (which is part of Old Guard Japanese business), I don't know if it's because the guys at the top of Nintendo have been there forever and don't know any other way, or if it's because the people leading Nintendo just aren't that good at running a business, but the burden is fully on Nintendo to make third-parties attracted to their platform. No one else is responsible for the success of your business except you.
