Closing Discussions

Why is is that some discussions get closed after just a few pages whilst others are allowed to run and run seemingly for ever?

I can understand the moderator having to take action if a discussion becomes abusive or breaches the community guidelines in some other way. The thread "Why are so few CC members posting on CT?" has been closed because according to the moderator "I think it has been pretty well covered and people have had the opportunity to share their thoughts and ideas". At the same time the thread about Sunday Departures now has over 1600 postings and it is still continuing.

Where is the logic or consistency in these decisions?