It was because this thread allowed some of users to get the last few merit ^{[10 or 20 for example]} to rank up. Most of them get some merit from various other threads and post a countdown to the next rank when it gets really close.

Some examples:



Why do you think that's the problem, even I don't think they are obliged to continue sharing merit in this thread if they don't find suitable users. Although they gained the last few merit to rank up in this thread, but disappearing or returning to this thread is not an mandatory.