A big issue of stadia, and a huge advantage for GeForce Now, was the environment they provided for the game.

GeForce Now runs essentially normal Windows VMs with GPU passthrough to some of their beefier multiuser cards and a custom remote desktop mechanism. There's no special porting involved, it's essentially the same setup, but extra optimised, as what people setup themselves using various cloud providers.

Thus GeForce Now has close to zero porting burden and could just attach Windows Steam client integration.

Stadia didn't have that - I have no idea how it was actually implemented, but someone mentioned that Stadia version of Cyberpunk 2077 was "the PC version, no porting".

Except the Stadia version is the otherwise unreleased Linux port of the game, using Vulkan for rendering (unlike DirectX on normal PC release). It had to use a custom input, sound, etc implementation for whatever interfaces Stadia provided.

FWIW, AFAIK that build also worked on normal Linux with X11 so long as you had Vulkan drivers.

But yes, the big hurdle was that Stadia required a Linux binary, possibly also required Vulkan. Every game they wanted to add had to be effectively ported.