I suspect if any game developer made a game like SAO (non-VR), they would want to get license rights to have the built in audience. The would also probably only be able to do 20 levels/regions at most. As a long time WoW player, making a decent MMO is very hard.
The biggest problem I see with an actual SAO MMO would be the lack of detail from the manga and shows. Actual MMOs require about 100 times more depth and breadth than most shows give to an actual playable virtual world.