[esskay]

Because Steam Workshop is only for games on Steam's shop, distributed by valve. That locks out huge chunks of your audience. If anything its good thats not taken off otherwise we'd have EpicMods, GogMods, etc and even more fragmentation.

Sure it makes it easy for people who own the game on Steam, but what about everyone else?

Also, as popular as Steam is, their workshop section is pretty awful. The search seems to have been cobbled together and has awful matching, the discoverability of mods is dire, and they're still using the same antiquated poor UX for discussions as they were over a decade ago.