Well running servers for online games I'm sure costs money. I think armor games has more things to worry about than running online servers for flash games.
I can't wait for it either, as it'll make Raze much more fun for me.
It isn't possible for every single flash game to have MP. Since multiplayer is an expensive investment, such games are infrequent. You can't just will for more of them to pop up. John has the privilege of getting Exit Path to be multiplayer, because he is backed by AG and Daniel McNeely. It's hard for your typical devs to make such games without some financial support.
I'm personally hoping Chaos Faction 3 is made in multiplayer. It'll be years though.
