Other than the broad “Microsoft hate” people is thinking about, there are two reason I can think about; and none about Gaben being a Microsoft’s scorned ex-employer.
conflict of interest: Microsoft’s EEE. Look it up. If they rule the OS on which majority of Steam Hardware work on, Microsoft rules on a competitors against it’s “Xbox experience”.
Microsoft doesn’t allow open source drivers to officially run on Windows. Any hardware driver must be done, paid, by close doors at Valve and no extra help as the whole community around mesa and affiliated. There’s a reason why Valve is skipping Nvidia on basically everything.
Other than the broad “Microsoft hate” people is thinking about, there are two reason I can think about; and none about Gaben being a Microsoft’s scorned ex-employer.