It's not the usual type of thing we hear, in fact the developer actually gave out some interesting details.
Here's what they said:
As for the actual Linux build, basically, things are so far not very good with Linux runtime in software used (GameMaker: Studio),
- Gamepads may or may not work because there's a limited (embedded) database for them instead of having an editable config file like SDL does.
- Mice can be detected as gamepads (I think because some distros register a generic device + mouse# device for them?).
- Depending on undetermined factors (library combinations?), audio may cease to play.
- Depending on desktop environment, different interesting things happen with graphics - ranging from garbage data outside the game area (i.e. on empty "letterbox" areas) to actual texture corruption or literally having half of game screen missing for no good reason.
- Depending on more factors (GPU drivers?), other interesting things happen with graphics - e.g. delays can be required when changing between window and fullscreen or display buffer ceases to change size or becomes garbled.
- Unless running via a .sh (LD_LIBRARY_PATH=./lib:$LD_LIBRARY_PATH DISPLAY=:0 stdbuf -i0 -o0 -e0 ./MainBinary), native extensions (.so) cease to load correctly.
- Compiled Steam builds are broken by default and will not hook up with Steam API unless moving files around and adding a steam_appid.txt (which is not supposed to be shipped with release builds).
- Related or not related to above two, sometimes Steam and/or Steam P2P native extensions cease to load for some users.
- When working with Steam Workshop, some users have GMS games unable to access downloaded items (permission issues?).
Overall it's a pretty tough decision to start making Linux builds if you know that they won't work for many people and you won't be able to do anything about that yourself (aside of waiting).
Hopefully things improve on this front by the time when current tasks on the game are finished.
That's quite a list of issues, although the one about running it via an .sh file isn't what I would say is an actual issue. Lots of Linux games use this method, including most Linux games available from GOG. I'm pretty sure all Feral games use that method too.
The other reasons are all likely quite valid and depend on GameMaker.
A shame, but I really hope they manage to pull it off. It certainly looks like a game I would enjoy, especially as it looks completely nuts.
Perhaps one of our lovely porters like Icculus, Ethan Lee, Knockout Games or someone else could arrange a port.
Thanks for letting me know, Luke!