It's also possible that developers wait to be paid if they release for an extra platform. I don't mean sales money, but just extra money. Google made Stadia? They pay developers to release there. Desktop Linux has more users, but no one is paying them to release there, so they are ignoring it. If Valve acted more like a steward of Linux gaming, it could get better. They kind of tried in the early days of SteamOS, but then they stopped this marketing push. And despite the size of the Linux gaming market and user base growing, we are still ignored by many developers because of the above.
Exactly, Stadia gave Larian a pile of cash to make sure BG3 was ported to Stadia, with the condition that certain BG3 Stadia features would be ready by certain deadline, that is it, no other requirements. It's why they've been able to make BG3 an AAA game without draining their bank accounts and while staying independent. Larian did a lot of it on Google's dime in exchange for porting it to Stadia with no other demands at all. Plus Google has already spent alot on advertizing Baldur's Gate 3 on Stadia, so that is free advertizing for Larian Studios.
