Found this article. I thought it was a pretty interesting read. Maybe you will too...
Just a little snippet from the article in case you don't feel like reading it all..
"Mark Kern, the original lead developer of the breakthrough MMO World of Warcraft and now the head of Red 5 Studios, explained the problem in an interview with Wired earlier this year.
“The biggest problem is the fact that you’ve got a monthly model, but it’s so expensive to make content for the traditional MMO now that if you spend $250 million like EA did on Star Wars, you’ve only got 30 days’ worth of content,” he said. Old Republic’s big draw was an elaborate, multi-part storyline set in George Lucas’ galaxy far, far away. But once Old Republic players burned through all of that content, there was nothing to keep them involved. The cost and time required to develop more content for players to enjoy made it difficult to impossible to keep them supplied with entertainment for their $15 a month.
“So you’ve made 15 bucks from the consumers, basically, before they churn out of your game,” Kern said.
World of Warcraft, said Kern, didn’t have this problem because in 2004, game production costs (and players’ expectations) were so much lower that it was economically feasible to churn out content and keep players engaged every month, so they didn’t cancel their subscriptions.
With today’s high cost of game production, Kern said, “that model is dead.”"