About cross-platform and my point of view...
The main problem seems the Delphi and DirectX stuff.
Let's start with the simplest one, the DX, simple don't exist an excuse for them not ported the code for OpenGL, this is something that they should have done years ago, even if for now they are locked on Windows. Using and maintaining a OS specific 3D language/API when we have OpenGL
About Delphi, where to start???
So Borland liked Pascal and wanted a object oriented language and Delphi born; so it was a new stuff only of Borland but nonetheless many software used it, today I'm still surprised of it's success, being it a so locked thing.
For years so many code was wrote that make it almost a no go porting it, it's what I think happened with Altium...
Back in the late 90s early 2000s I was also one of those sold out by Borland propaganda and all the promises of Delphi and C++ Builder going cross-platform, at least Linux; several years ago just for fun I exchanged some messages with I think at the time CodeGear(Borland -> CodeGear) about all those promises and they continue saying me that support for Linux were around the corner...
But guess what, now at 2014, let's say 15 years later and no Linux support
WTF
This don't affect me any more because many years ago I decided that I would not develop any more software that was OS locked.
Maybe Altium also listened the "Borland propaganda"