There is a lot, and I mean a lot of speculation, research, time and money poured into the question of why people leave a game in droves all of a sudden. But the amount of time it takes is fairly consistent, about 1 month.
Thats the time frame any game really has to see that huge population boom, a month.
It kinda begs the argument to be made, why invest in server technology for people who aren't going to stick around and play? You already got their money, they had the experience of play, they move on.
Some have described this as the rise of the casual low risk high reward kind of game play. Many marketers are pumping money into that part of the industry along with industry giants to try and make a killer profit on what use to be considered, and justly named, "Shovel ware".
Muse themselves have even made some killer mobile products for cash injection,
http://www.creavures.com/ is a good example of high quality art paired with a simple but entertaining mechanic. But it's designed for limited interaction, play it for awhile, forget it. We've got a huge generation of gamers who have been raised with that principle and they've got tons of choices to go and play with.
It's no longer the days of Baldurs Gate being one of 4 CRPG's that would release in a 2 year span.