Why does Steam/Valve care so much about Linux? I know as devs we all would prefer to use Linux/Unix. But developer experience isn’t a good business justification.
You can basically tailor the OS specifically for the device and remove unneeded bloat. Also the threat of Microsoft and Windows as mentioned by other users. The introduction of the Microsoft Store with Windows 8 basically kicked off this whole move for Valve. While it took over a decade of work, its paying great dividends now.