Multiplayer games require quite a bit of programming and server. The only people who can do that easily are those that work at Armor Games, who are good with programming in ActionScript and can get the funding from Armor Games to keep a live and reliable server.
You've also got to have a good concept that fits with multiplayer. For example, John's game Treadmillasaurus Rex would be a difficult game to implement multiplayer into.
The chances of multiplayer games being released regularly is pretty low.
