Probably because building equivalent applications on Linux takes 10x longer than using professional tools like Visual Studio on Windows.
Maintaining and supporting software on Linux is also 10x more expensive because you have to deal with many different versions of OS each that might or might not support various tools, as well as various different desktops and tools they may ...or may not support. Dealing with all kinds of possible version mismatch problems between different Java releases... open source projects etc... and you can say well just use Maven! But then after going through all the trouble you later find out Maven doesn't really work and you still end up with version mismatches in your dependencies.
And then to top it all off the market share of the Linux users is < 5% and so the amount of money that would be made from them would not come even close to the cost of building and maintaining a special version just for them.
Just a guess though... from my personal experience with developing software with Linux vs Windows.
You will be surprised how many developers work on unix based architectures such a linux or mac.
Isnt QT a good candidate to do linux variations of work done in windows with visual studio ?
QT is also available for windows. (free too)
If I am not mistaken, if development would move to QT, lots of other architectures are in hands range for exports. :)
Sadly, it would take quite some changes for ashley I supose.
The amount of upside you get from being able to distribute on near all platforms will surely outweigh the costs.
edit: its like with construct 2, teh ability to export to so many platforms, one of its geatest assests.