I have a couple of hypothesis that I'm winding in to a theory which I'm pretty confident will bear out for many players.
1. Spoiled for choice: We have literally more games than a person with unlimited funds and free time can play. My Steam catalogue (never mind other games on top of that) has ~40% of the games I've never literally run. Either games in a package deal that didn't really interest me or games that were uber cheap (DAMN YOU SUMMER SALE!) that I picked up at the time thinking I'll play later. These include Saints Row 3, Batman Arkham City and LA Noire. Being spoiled for choice makes us more discerning and less willing to accept pedestrian products that we previously would have played to death when there were far less options. And my steam account at 330 odd games is relatively small compared to some.
2. F2P takes away the need to justify paying a monthly sub by playing. I know that there were times in WoW I was only hanging around for the community, but since I was paying 15 bucks a month I should really be getting my monies worth... So I would invent my own fun like solo'ing as much as possible as a 60 druid back in vanilla, for example, rather than just letting the account lapse. Currently, with no sub, I have characters/accounts in EQII, LOTRO, DDO, Firefall, Dragon's Nest, Planetside 2, MWO, WoT, GW2, TSW (lifetime), most of which I'm not playing currently (PS2, MWO and some Firefall are about it). And EQII and LOTRO have had recent entire expansions released...
3. Very little is actually new anymore. Even GW2 with it's refinements isn't a new take on the genre, it's entire questing system is based on Warhammer Online. You remember first logging in to EQ or SWG back in the day, you had a game that was magnitudes larger than anything else you had ever played in most cases. I spent my first hour in WoW wandering around the Teldrassil noob area amazed at just how amazing the graphics were... ; ) Nowdays, I log in, hit the ground running, pick up as many quests as possible and start slaughtering. I have streamlined my "new mmo" experience to maximise time and minimise downtime even though I don't think of myself as a hardcore min/maxer.
4. Most MMO content is repetition/grinding. They can try and dress it up as much as possible but the end result is that you spend a disproportionate amount of time chasing gear or faction points etc rather than actually adventuring. DDO is one of the few games where this isn't so bad (there are a huge number of available adventures when you're leveling and you don't typically need to grind unless you're going for achievements) and even that get's repetitive.
Combine repetitive retread games with no sub (no impetus to play to get value) and a plethora of other choices, it becomes incredibly easy to become disenfranchised/burnout with a title much sooner than we used to, or to jump between games rather than dedicate time to one. At least, this is my experience.
Against this is the community effect of MP games where the enjoyment is derived from the social interaction in game rather than the game itself, ie. teamspeak banter or taking on harder challenges in a group of friends which can cause a game to hold your interest for longer than it really should purely because of the people you are playing with.
It took me a long time to come to this realisation and when I did, I started to stop playing games when I got bored with them rather than enduring something I wasn't enjoying. Probably one of the reasons that despite getting a lvl 90 something hardcore paladin in Diablo II, I couldn't even force myself past lvl 10 of the demo of Diablo III...
Asmodai aka Saviourself aka Stabbitha aka Azshadoria... or Azsh... Azshaz?