And why delete anything? Surely they are not running out of space. I'll volunteer $100 so they can buy another Terabyte to store those deleted discussions.
It works like this: The oldsters perceive that they enjoy a certain elite status by virtue of how long they've been there, and begin to think of themselves as the "keepers" of the community. They also develop a measure of contempt for newbies, perhaps because they (being new) want to talk about things that the oldsters have had a chance to discuss many times before. And perhaps because they (being new) represent a slightly different culture, which triggers stereotypical "kids these days" instincts.
So they begin taking measures to protect the site from junior members. A positive spin is put on it by calling it "upholding community standards", but it would be just as fair to negatively characterize it as "telling those damn kids to get off my lawn." I suppose which one you think it is depends on whether you're the one yelling or being yelled at.
Either way, the terminal point is that everyone has gone to have fun somewhere else, essentially because that's precisely what they had been told to do, and all that's left is a hoary old collection of ossified cucumbers fondly remembering how much fun they used to have and wondering where the magic went.
At an old-fashioned web forum I used to run we toyed with the idea of closing down and archiving old threads after they reached a certain age. The idea was to try and codify our opinion that old conversations might be worth preserving for posterity, but they aren't a substitute for new conversations and shouldn't be used as an excuse to stifle them. Never actually got around to implementing it, though.
In the early days of stackoverflow I seem to recall they had a policy of keeping similar or even duplicate questions. The reasoning was that the extra interpretations of the same question made it easier to find them in a google search. What they have now seems entirely the opposite, content being removed to the point where it doesn't exist in any form. I don't really care if someone thinks a piece of content is on topic or relevant, if it isn't relevant to me I just won't look at it.