The problem with small communities is that you don't necessarily have enough competent and available people to run the system reliably. Or worse, the people in charge leave, sometimes taking the servers with them, and no one else knows how to maintain it.

That's why Reddit and Discord are so successful. You can create your small community for close to zero cost and zero technical expertise, no one is going to pull the plug on the server because the one guy responsible for it didn't pay the bill, if the server in question is not an old PC in somebody's basement with no backup.

Even with good tools, building a community server still relies on an individual or a small subgroup to make an investment in money and time for the entire community, and if they stop doing so and no one wants to take over, the server is gone. On a platform like Discord, as long as there is any one person in there, it will continue running, the platform keeps the community accessible and does basic administration like spam filtering for free, even when there is no one to care, it can do that because it pays itself on user access, though premium membership and ads.