The problem is that:
1) Too many dolts and teens play online
2) Even if the players are intelligent, it's hard to motivate them to fall into some rigid structure. I real life middle ages, you see, there is a lot of motivation - physical violence, hunger, fear of God, stuff like that. Moreover, everyone has only one life. So people obey. But online? No chance.
3) Roleplaying abilities of different players. Each time an archaic mam-at-arms in some online game says "OMG, Lol, WTF?", my sense of immersion is totally destroyed.
Therefore it is all just a waste of time, money and effort.