Comment by saghm

The key difference is that Valve isn't trying to make Windows work, just desktop gaming, which I'd imagine is a large part of why Microsoft's efforts failed. As much as Linux desktops haven't particularly had much polish over the years, there's still an advantage to taking something bare bones and trying to flesh it out in a way that's works well compared to taking something that's already pretty bloated and then trying to retrofit it into something new.