Over-saturation is a key element. Too many games, not enough people to properly fill them all up, and keep them going ingame or more importantly, financially.
People like their little slumps or ruts. They then stick to one MMO (this happens in competitive gamers as well I have experienced), and seldom handle change well.
Also too many developers / publishers / or whatever trying to cash in on the popularity that is WoW. This has lead to some seriously subpar games being released, and actually some great ones too, but they often are lacking the development skills and/or proper longterm funding.
Then there is also the influx of the more 'casual' gamer into the MMO scene. Before MMO's almost required that you had no job, no life, or responsibilities so you could get ahead in this genre. With the advent of WoW that has largely changed, and with that change came an influx of people with shorter attention spans (*cough* larch <3), and they are seldom happy with almost anything, even their game of choice.
