I've also been wondering: Linux versions missing on GOG while available elsewhere seems to be a recurring phenomenon.
I am guessing adapting the Linux versions to conform with GOG standards (to fit the installer and whatnot) requires some additional effort from the developer, whereas on HumbleBundle they can just dump some self extracting .sh file for a Linux version, which many developers do rather than build proper packages.
On Steam, Linux versions might be more prevalent because packaging is more consistent across different operating systems, or Valve provides more assistance, or maybe the developer simply deems the platform more commercially viable.
But, of course, that's all pure guesswork. It would be nice to get someone involved to comment, especially on how much effort it is to ship an existing Linux version for GOG.