They're using stuff [1] that Mono doesn't implement [2].

[1] https://github.com/SonyWWS/ATF/tree/master/Framework/Atf.Gui...

[2] http://www.mono-project.com/WPF

Since this is related to tools surrounding games there probably isn't much point to make it really cross-platform. If your designers and developers are using Windows anyway investing resources in a Linux version doesn't help anyone. The games themselves run on other platforms anyway and don't care where the levels were designed.

A few things that won't run on Mono are pointed out in the readme as well: »You must have Windows 7 or later to run the circuit, FSM, statechart, and timeline editors, due to their use of Direct2D.«

Thanks for pointing out the Windows dependencies from the page!

Lots of people are doing game dev on Mac and Linux, though not so much on the big game studio dept. For example you can do Unity dev (& target all platforms) + iOS dev + Android dev on a Mac pretty well.

Well, this is Sony open-sourcing an until-now internal tool. Which means that up until now their only interest was to satisfy their own developers. Who are most likely on Windows.

AAA game studios are mostly Windows based, specially given the way gaming culture works, so most probably no one ever tried anything on Mono.

Additionally, the readme also has references to DirectX dependencies.