Why Windows only?
This game is very popular among Mac and Linux gamers.

Which Unreal game do you mean? At least the GOG version of UT 2004 is relatively easy to run on Linux natively.
The real answer to the question is that GOG does not support Linux atm, unfortunately.
And the Mac version will probably come when the game installers will be updated to GOGs new one. At least that happened with many other games.

It would seem unlikely trying to get the linux version of ut (apart from gog doesn't support linux yet ) because trying to get the loki version of ut99 work is so much work and it seems to get harder with each newer version of the kernel.

I believe the OSX version of Unreal exists by the developer of the 227 patch at oldunreal.com but Epic Games wouldn't let him release it because of "not enough bandwidth" or something like that. Same thing when he wanted to update UT99 and Unreal II as well.

Ah, I did not know that there were Loki ports of UT99 and Unreal 1, I only played UT 2003 and 2004 on Linux, and that had more official ports.
But it makes sense that the Loki stuff gets more and more difficult to run with new kernels and libraries.

I don't care that much about Mac OS X (never owned any Apple devices and probably never will because i really don't like the company and OS), but everyone should be able to play this great old games, so I hope that guy finds a way to fix the Mac versions and distribute the patches. Maybe he could ask GOG about that, I think they would not mind someone helping them getting the Mac versions to run.
But maybe GOG does not have the rights to the Mac version, as oftentimes Mac ports were made and published by other companies then the original developer/publisher, and retain rights to that versions. Like many Linux port.

*Sigh* it would be great if somebody would find an easy way to fix Loki and other old Linux and Mac ports. And some of that publishing rights would be sorted out.
Oh and I don't know about Mac ports, but Epic does not seem to care about their Linux ports any more, when I was getting the GOG version of UT 2004 wo work on Linux, I noticed that Epic does not host the Linux stuff any more. Fortunately Icculus hosts the patches and ECE pack and such stuff now.