It's because eventually you either 1) "Beat" the MMO you're playing (it takes months for a given MMO to come out with new content, usually the hold-me-over stuff until then is repetitive and boring), or 2) You become deeply pissed off with the product you're currently playing as you take a closer look into its dysfunctions that will never be addressed by its owners. Once that happens, players want a new experience. There's also some nostalgia chasing involved as a game you've fallen in love with mutates around you over time.
It's because eventually you either 1) "Beat" the MMO you're playing (it takes months for a given MMO to come out with new content, usually the hold-me-over stuff until then is repetitive and boring), or 2) You become deeply pissed off with the product you're currently playing as you take a closer look into its dysfunctions that will never be addressed by its owners. Once that happens, players want a new experience. There's also some nostalgia chasing involved as a game you've fallen in love with mutates around you over time.