Wait, are you serious?
I mean,
and
https://www.curseforge.com/
Not to mention dozens of smaller sites that, unlike these two, have no auto-updating utility.
Out of these three I actually prefer Nexus. Surely it has its own flaws. But there was an attempt to create a similar tracking/updating utility for SH.
On the size of community, I agree. I understand I tend to be biased here, but I view things as an occasional modder myself, and a repo without version control… eugh. As a modder I need a bug tracker and git push more than I need discussion (and definitely more than I need ratings).
It may be useful strictly for stable releases, which is… problematic… when the game itself is in Early Access stage and updates frequently. Unlike games, there is no “unstable” or “beta” branch for mods (actually, there are no branches at all - correct me if I’m wrong), which is the effect of Steam’s “one game, one version” policy. Meaning, you either release a mod for latest stable version, or post the same mod twice, one for each version.
Then there’s a question of licensing and source code which is especially useful for resource exchange within modding community.
Long story short, Steam is mostly convenient for casual mod users, not hardcore mod developers 
Still, it’s better than nothing. I imagine many people will use it.