Steam for Linux was started by ex-Microsoft developers
The information comes from Richard Geldreich, part of the Linux team who left Valve back in 2014. On Twitter, Geldreich interestingly said this:
What isn't commonly known is that the original Steam Linux effort was started and led by a number of ex-Microsoft employees who, for various reasons, believed that Windows was going in the wrong direction.
Definitely something I didn't know. It sort of makes sense though, with many in the gaming and tech industry showing concern on where Microsoft was taking Windows. Valve's Gabe Newell (who also worked at Microsoft) famously said Windows 8 was “a catastrophe for everyone in the PC space” and that getting their games and Steam onto Linux was “a hedging strategy”. From the big touch orientated interface that tried dropping the traditional desktop, to the hinted plans at pushing people towards the Windows Store there was a lot of ire aimed at Windows 8.
Getting Steam on Linux, and the original Valve blog posts like the “Faster Zombies!” lit a fire under Microsoft, clearly feeling a little threatened (or perhaps just motivated to do something) as Geldreich actually wrote on his blog back in 2017 that Microsoft ended up paying Valve a visit.
Without Valve's initial push into Linux, and their continued support with various projects and contracts with developers to work on improving all manner of things like the Linux Kernel, Mesa drivers and more, Linux gaming would have gone nowhere. It's still a tiny niche now of course, but it's come a very long way in terms of usability and performance.