Tech issues. The only way to do it would be to host a central server. Then figure out a way to get Steam, Gamespy, PSN, and Xbox Live to all play nice.

Nothing coming down the pipe with the exception of FFXIV is true crossplatform for this reason. There are too many people to pay, organize with, and work around. In fact FFXIV is NOT going to Xbox in part because of this.

FFXI was a major exception on everyone's part. Playonline was the work around (a private network that all others could connect to. And you see what a steaming pile THAT was. SE is killing it).