Users flood the community with low-effort content to karma farm
That's where the mods kick in. That's why Askhistorians are awesome and some other subs are not.
Subs die or prevail with the mods at hand. If the users grow, but the mods do not, and it becomes too much for the mods to handle, it will fall. It's easy logic.
The problem isn't the quantity, the problem is moderation in regards to the quantity of the userbase.