I'm going to quote our Founder and Programmer, John who answered a similar question in our Reddit AMA, a few months ago:
"The problem is, the external developer that did our Linux porting went out of business right after it launched, and before we could fix all its issues, tidy it up, and update its supporting technology. There's no way to get a refund or a financial remedy because, well, they ran out of cash. To extend the Linux build to GOG, we'd have to be able to make a new build (Steam builds won't run on GOG), and we don't have the developer or budget available to do that.
Financially speaking, the total amount of income we've made from our Linux port on Steam after 2 years on the market is about 1/4 the cost of porting it.
Why is it so expensive and difficult? Because of the underlying technology we chose as a framework. The game rests on Adobe AIR, which provides platform level abstractions like low level graphics, file IO, networking, etc... At the time we started development, Adobe was strongly supporting Linux. Adobe then abandoned Linux and left both the developers and the players flapping in the breeze. Our Linux port takes essentially the same approach as our console ports, removing Adobe AIR completely from the equation.
Hope that sheds some light on it."