Google's reputation for not supporting things long term is finally starting to affect them in noticeable ways. Developers didn't onboard because they were afraid it would get shut down. It got shut down because no one onboarded.
Developers didn't onboard because it was Vulkan-only. You can release for GOG as an afterthought, but you had to build for Stadia as Another Separate Platform, and they somehow managed to make their SDK requirements more onerous than Xbox's. They might still of joined if there were any customers, but customers didn't join because of the prospect of needing to buy their existing games again, then pay for a subscriptio…
While Sony, Microsoft, AMD, Intel, NVidia do cool tech sessions, Google is all about analytics and Play Store.
Then they expect developers used to devkits and Visual Studio plugins, to use classical UNIX like development experience to target Stadia, while hoping Stadia will stay around.