If we're being realistic from a business standpoint: Linux is at best, 3% market share. A very passionate 3%, but 3%. Using resources to support such a niche sector is a hard sell.
This argument would be a great one in 2016.
Now though, proton/wine works more or less for everything, and the storefront is a web based one anyway.
It's a glorified wrapper around curl, wine and a webview, a few interns could knock this out in a few months. For "3% market share" (growing every day, thanks to Valve) its a no brainer, but Sweeney has no brain.
3% of millions of people is a massive number of people. Given how easy recent work on wine has made porting from windows, it's really hard to defend not having a linux version, from a business standpoint.