Further searching turns up the information that "federated" Bluesky PDS instances are limited to ten user accounts each, and API usage limits which may constrain things further. So that would explain why there aren't any big ones.

So far as I can tell they do all still "federate" through the central server, not directly with each other. So there being not much point in it may also explain why it hasn't caught on.

Almost as bad as Threads, really.