Theres other games RTS style on 360 that are apparently popular. One of my buddies is really into C&C. Big issue with an mmo on the xbox network is they want to run the servers on their network. So anyone who wanted to play it on 360 would likely be segregated from their friends that play on pc.
That, coupled with extra costs in porting code, Microsoft fees and requirements. A small company like reverie could see any profits for hard work disappear or even run in deficit to keep servers together. If they allowed it to be split and MS have their version of the servers it would have a negative impact on player counts.
Staying exclusively PC will allow Reverie to keep costs down, troubleshooting & development easier, allow for better graphics/UI , faster updates, more content expansions sooner. It might be a good idea to port to xbox after a successful year or two.
IMO Reverie being a smaller company doesn't have access to some of the assets bigger companies might, and overspending on that kind of a level can be enough to kill a game. I don't think any of us want that lol.