many reasons why they would stop posting such as getting annoyed with others because on nostr telling them the truth is possible without getting banned, and that's something people aren't used it, so they are forced to hear the truth and it hurts them, or
maybe they got bored, social media gets boring after a while, there's more fun things to do in real life like party with friends or,
maybe they are taking a break, or many other reasons but it's actually more organic then X and replies are more then X,
nostr rocks regardless if they left
