Yoshi P explained what happened.

When a world is at user cap, the lobby server waits until 100 spaces have cleared up. Then it checks again, since people are returning from duties all the time, and allows however many of the next batch of people were in the queue to get in.

When the world is full and the queue doesn't seem to be moving, it's because you're literally waiting for enough people to join a duty, log off, or lose their connection to free up spaces.

Large scale instance duties like Bozja actually could help clear out some of the server overhead, but there aren't enough people doing that content any more since everyone wants to be in the overworld for MSQ.