I had a conversation with Aristox about "why people quit" 12 years ago back when this MUD averaged 130 players during 18 of the days 24 hours. The last 6 hours would have had 70-80 players. My theory then was that people are quitting [or will quit] because the imms keep making content harder. Imms (or changes implemented by imms) keep forcing people to join larger and larger clans. They implemented a legendary system that requires 30 million experience to gain 1 level (back when it was hard to get, and you'd lose ~20 million for dying), which meant that anyone who wanted to gain legendary ranks had to join a large clan. Aristox told me that he didn't like large clans, that he didn't want people teaming up into 40-50 man squads, and that he didn't want the MUD controlled by 2 factions. And he said he made the game harder to discourage that. I told him it only ENCOURAGED that because the only way to get the best stuff would be to be IN those 40-50 man squads because everything was so damn hard to do. Anyway, in the past, I would speculate that people quit because the game was too difficult. When the game averaged 170-180 players, the beauty was that you, and 4-5 friends, could do almost any boss in the game.
Unfortunately, to "discourage" large clans, imms made the game harder and harder, which actually had the opposite effect (ie, creating large clans, making it less fun for everyone).
I think people quit today for a variety of reasons unrelated to the game .. games with better graphics, family commitments, boredom, etc. But I think people quit in the past because of a policy put in place by the administering staff whereby only the largest, best equipped groups could get to 40% of the end game content. Stupid-effing-policy.