There are a couple of reasons, although you can actually still play it via linux and I'd be happy to walk you through it if you like.
Ok so basically, there is a pretty big issue with infrastructure. Support for our original lobby ended (as in the lobby protocol changed and our lobby was deprecated), so we had to switch to a much nicer, much newer lobby. The problem is that this lobby requires the use of a launcher which does not support the game staying in it's install directory.
On windows there is a large issue with programs shoving EVERYTHING into appdata. This is not only stupid, it's extremely problematic, because appdata is hidden from users. Of course if you are aware of it you can access it, but the vast majority of windows users will be oblivious to it. Meaning that appdata becomes very bloated over time with leftover shit from programs that have been uninstalled long past.
I raised absolute hell over this and absolutely refused to allow evo to get sucked into this shitty practice. That insistence on doing things the right way unfortunately didn't carry over to linux. I don't develop the launcher, so I really have little say in it other than to bitch at the developer who does work on it. Welcome to Open Source.
Steam explicitly forbids placing files outside of the program install folder (and doing so is some serious asshole design). A portable launcher for linux has not been developed, so frankly, I can't include it.
That said, I can't help you get it running on linux. However, this comes with the explicit understanding that it will intermingle with anything else you have installed that uses the spring engine. This likely will cause 0 issues, but the disclaimer stands.
You can get it here: https://github.com/EvolutionRTS/EvoRTS-Chobby
Just scroll down until you see the download links section and click the linux one. You won't have the music with this option. However, if you do want the music, just tell me here and I'll make a quick guide on how to add it (it's pretty easy).