My theory is that back in the EQ days it was easy to play with the same group of friends for many hours each week for a couple years because there wasn't much else out there. Now there just TONS of good quality games, many of which are free, that scratch any number of itches.
So between everyone having less time and more responsibilities than we did 14 years ago there are also so many directions everyone's interests are being pulled.
I absolutely miss having 3 or 4 (or more!) people I could count on spending time with almost every night. Now that'll happen for a couple weeks when new games come out but then everyone just kind of scatters to the winds until the next game comes out in a few months.
It's just too difficult to find a game now that attracts everyone the same amount. Back when it was UO, Meridian 59, Asheron's Call, or EQ it was a piece of cake. 2001 Dark Age of Camelot and Anarchy Online comes along and there's a split but still a small enough pool of games to keep most people together. Then 2002 Final Fantasy XI, Earth & Beyond, and Asheron's Call 2 come out and the population spreads even thinner.
2003 EVE online, Star Wars Galaxies, Horizons... 2004 City of Heroes, World of Warcraft, EQ2... 2005 Matrix Online, Guild Wars... 2006 DDO, Auto Assault... 2007 Vanguard, LotRO, Hellgate, Tabula Rasa... 2008 Age of Conan, Warhammer... Not to mention we start seeing more and more eastern games sprinkled in there. They just keep coming! 2009 Aion, Champions Online... 2010 Global Agenda, Star Trek Online, an explosion of lesser titles and F2P... 2011 DC Universe Online, RIFT, another explosion of lesser titles and even more F2P... 2012 Tera, The Secret World, GW2, another explosion of lesser titles and even MORE F2P...
How is a person to settle on just one game now?? Pretty much everything is now F2P in some form except for just a small number of games. And who knows how long it'll be before they switch over as well. It's just too tempting now to try other games.