Sad to see this, another developer hitting SteamOS-specific issues where Valve didn't approve the Linux build. This time it's the free RTS Zero-K.
Speaking on the Steam forum about why this happened, the developer said this:
Unfortunately, we have been unable to produce a Linux build that passes all the Steam quality control tests. We have decided to release without Linux as the alternative at this point is to delay the release for some unknown length of time. The standalone installer still works and syncs with online multiplayer so it is not like we are dropping Linux completely.
We aim to add support after release. This would be helped along by linux testing and support from within the community.
Reading further into it, another developer commented about it being specifically SteamOS "The problem was with SteamOS big picture mode where the window would never show.".
For now, if you just want to play it you can grab it from itch.io. If you feel you can help them solve their issues, do let them know in this Steam forum post. Would be great to see it get proper Linux support on Steam, it's a really fun game, especially for those who like the more traditional base-building and large army strategy games.