For a lot of us here, we spent most of our childhoods playing mmorpgs. Be it Warcarft, Everquest, or Runescape, we played a lot of it. However, as we grew older, they stopped being a huge interest for us. As this has recently happened to me over the last year as I went through my junior year in college, I started to put thought into theories of why a game genre that used to literally be *the* only genre I played, now doesn't interest me really at all.
Then it hit me:
When we're kids, we aren't really making a living yet. Yeah we're having fun, but a lot of us lack a sense of purpose or get bored just sitting around after school watching tv when it wasn't nice out enough to play. I feel like this is what drew a lot of us into these games as kids: we took the role of a grown adult, and assumed their jobs, their duties, their lives, and we pretty much were given an avenue to live life as an adult with a purpose. It gave us a life of an adult as we were kids.
That's why my theory is that as we grow older and actually become adults, we feel no need to assume the roles of a fictional character that requires so much attention or maintenance, because we now ARE those adults that require attention and maintenance xD We have jobs we have to do, we have bills we have to pay.. we don't quite have dragons to slay but we can do that in a different game genre that doesn't require nearly as much of a time investment.
I dunno, just some thoughts I notice there are a lot of people my age suddenly completely uninterested in a genre they would spend hours a day playing with no break. This seems like a pretty logical explanation to me.