I have never understood that...

Because Windows has a bigger market share than Mac or Linux.

LOL. Soundtracks are MP3, FLAC, etc. There's nothing platform-specific about them. The actual reason is that soundtracks are DLC, which means they inherit the platform(s) of the games they go with.

Soundtracks are DLC, and the system for DLC is copied from the base game. Here's an example of a soundtrack for all three operating systems: Ken Follett's The Pillars of the Earth - Soundtrack

Just one of the many bugs and shortcomings here that they have no intention to fix.

I could have sworn there was a soundtrack with Windows OS but the base game was Windows and Mac... But anyway, yes, soundtracks are "multi-os" regardless of what it says.

Here's one example that "inheriting" platforms is not always the case:

https://www.gog.com/en/game/vampire_the_masquerade_coteries_of_new_york
https://www.gog.com/en/game/vampire_the_masquerade_coteries_of_new_york_soundtrack
same with artbook: https://www.gog.com/en/game/vampire_the_masquerade_coteries_of_new_york_artbook

Maybe Linux and Mac builds of game were added in later time or who knows.

Curiously, GOGdb says the system was changed from "none" to "Windows".

The main thing is that it's possible to have DLC with different OS requirements than the base game, but they just don't bother to do that. Even if they don't have a way to automate this, it wouldn't require a superhuman effort to mark soundtracks and artbooks manually.

I'm still pissed that I have to jump through hoops to download Galaxy on Linux because they can detect my OS, and decide I'm not allowed to download from their site even if I can make their client work in a Linux environment.

The reason is probably because they are offered as DLC where we are forced to own the base game before purchasing the music. And I don't understand that.

I am not even talking about that.

I guess you really need to think deep enough to understand.

In the first instance, a soundtrack is a DLC. That means the base game is usually required, and if that base game is only Windows, then yes you can say it is kind of Windows Only, though the files themselves, even the downloads (zip files) are suitable for use out-of-the-box with any common OS.

Whatever the state, it is likely determined by the publisher or developer and not GOG.

If you don't have the base game for a soundtrack DLC, then that would require GOG to create a non game entry in your library just for that soundtrack, and that just complicates things I guess. Sometimes we get soundtracks as a free goodies pack, and they get an entry in our personal GOG library. In any case, it requires more work by GOG and permission from the game provider, and I guess GOG have enough on their plate.

Most soundtracks are available from sites like Bandcamp and Soundcloud etc ... usually for a fee of course, just like GOG. GOG's main business is games, and game extras like soundtracks are a kind of bonus.

You should also consider, that soundtracks are often very cheap at GOG unlike other sites that specifically sell music, that is because they want you to buy the game at GOG, not just the soundtrack ... it's a promotional discount thing.