Linux just barely broke 3% share. As a company, whose goal is to make money, would you focus on what 97% of your base uses, or the 3%?
If your game is mobile friendly, treating Steam Deck not as an afterthought may be beneficial. Proton is not perfect. It has bugs, it loads a whole fake Windows environment into memory and API translation costs CPU and battery.
Further more, the company needs to spend QC resources for 1-2 versions of Windows, vs the multitude of Linux distros
That's completely wrong. For games, the developer only needs to target whatever the latest Steam Linux Runtime is. It's 100% identical across all distributions where the Linux version of Steam runs. That's its entire point. Steam Linux Runtime is a more stable target than playing catch up with yearly Proton releases.