Yes, subreddits really are communities: unique spaces populated by real humans and cultivated by actual human moderators. Each has its own microclimate and culture.

While in principle, you might assume certain topics should have a community of a certain size and caliber, there's no guarantee that such a community exists if the right set of humans haven't happened to coalesce around it.

That's just the nature of human group behavior. You might live in a city that has enough disco fans to support a thriving disco night every Saturday, but there's no guarantee that the right DJs and nightclub will get together to make it happen.