Free to Play/microtransaction MMO's haven't taken off over here like they have in Asia in part because the games that have gone free have generally been games that failed to take off when they cost money. The game has to be GOOD, or no one will pay to begin with.

There's also the economic issue of it. Which of the following would you rather have:

100K Subscribers
$15 monthly fee (1.5 million monthly income)

500K Players in a free-to-play game
10% of players (50K) choose to pay
Average paying customer spends $30/month (1.5 million)

If you're a gaming company, you'd almost certainly prefer the first option, cause the second means wrangling 5 times as many players (with associated bandwidth costs, server loads, etc) and having to figure out how to keep two player bases happy at the same time. You'd have to find some other way to get money out of your population (e.g. in-game advertising, which won't work in many fantasy game worlds), or be facing the imminent shutdown of your game, or it wouldn't be worth your time. In Asia, on the other hand, the amount of money most players can afford to pay as a monthly fee is much smaller (e.g. WoW costs literally pennies on the dollar). As a result, it's sometimes more profitable to collect as much money as possible from the big spenders (who will come in greater numbers to more popular games) than it is to collect a relatively small amount from more people.