Did you mean if modders themselves only upload to Steam workshop and not general download sites? I haven't researched that, but I will certainly look into whatever options we have to encourage mods to be open to everyone.
Here's the situation, as I see it (disclaimer: I have no Steam account, and I don't do mods. At most, I do patches/workarounds, which I publish from bitbucket):
If you have a built-in mod editor (or just a scenario/map editor), Steam users demand easy integration with Steam Workshop (e.g. a one-click upload to Workshop). Steam provides an API for doing so, and devs integrate this API into their games. Thus Steam users will most likely upload to Steam Workshop and nowhere else. To a lesser extent, having a one-click download and install in Steam workshop encourages modders to upload to Workshop as well. So far, there's nothing wrong with that; there may be APIs for Nexus, moddb, or some similar site, but I haven't looked into that.
Steam Workshop was created to serve Steam customers, and Valve probably doesn't like serving data to the rest of the world (although it's probably a negligible extra load, given that they are a near monopoly). They discourage allowing non-owners of the Steam game to download mods from the Workshop. The default behavior of a workshop is to disallow public downloads (actually there appear to be two separate restrictions: you must be an owner of the game, and you must be logged into Steam).
It is possible to disable both restrictions, as some games have done (see the list I linked to; in particular, the Shadowrun games started out restricted, and after customer complaints, they changed to unrestricted). Steam apparently does not document how to do this, so even developers/publishers who want to support outside users can't do this easily. I recall at least one who enabled non-owners, but failed to enable non-Steam-customers, so it worked with some web-based steam downloaders that actually used a Steam account in the background, but not on personal downloaders, like the greasemonkey-based ones.
As I see it, there are only three ways gog customers could be properly served in this situation: either no Steam integration (but that is a disservice to Steam customers), some sort of regular mirror onto Nexus or some such public site (supposedly that's what Neverwinter Nights EE does, but I haven't verified that), or figure out how to force Steam to make the workshop fully public. Some modders may mirror their own mods, but most won't, since that defeats the purpose of easy Workshop integration