As I explained in another topic, multiple parts of the Steam version, including the multiplayer, depend on Steam API, servers and features to function (game will crash the moment it can't connect to Steam and will not start without it) so only thing they could sell here would be Steam keys. Selling only on Steam also mean they don't have to deal with multiple stores and they only have to monitor feedback on Steam forums. Not supporting Mac/Linux means they only have to compile, test and maintain builds for single operating system and don't need to support multiple graphic renderers (DirectX 11 for Windows and OpenGL for Mac/Linux). They also don't need to have Mac/Linux PC's to test on in their office.
Do remember that they dropped Mac/Linux support during the development of the prototype and last version released for Mac/Linux was 1.04 while for Windows it was 1.05b.