The original reasoning for Valve's Linux push was the shock from how terrible Windows 8 was and for the most part still is.
At that point, they realised that if Microsoft is being stupid, there's nothing they can do against it. Almost their entire market is in the hand of Microsoft.
And yeah, it certainly didn't help that Windows 8 featured the Windows Store, therefore officially competed with Steam, and included a new application API (Metro Apps, now known as UWP), which I think back then was still exclusive to the Windows Store. Nowadays you technically can sideload them, but if we look at Android, that's still not much comfort.
At that point, they realised that if Microsoft is being stupid, there's nothing they can do against it. Almost their entire market is in the hand of Microsoft.
And yeah, it certainly didn't help that Windows 8 featured the Windows Store, therefore officially competed with Steam, and included a new application API (Metro Apps, now known as UWP), which I think back then was still exclusive to the Windows Store. Nowadays you technically can sideload them, but if we look at Android, that's still not much comfort.