Interesting question really.
The most obvious answer is for technical reasons. If you're going to display anything in split-screen, you're effectively doubling the viewing area, thus also doubling the memory load along with processing requirements.
Yes you could lower the graphical quality, but said optimization is also a pain because Reverie would basically have to go through every feature and tone it down specifically for split-screen play.
Also for gameplay reasons strategy suddenly becomes much less appealing when you can see exactly what your opponent is doing and vice versa. Having said that, network play should be more than possible across to 360 systems, but I'm afraid that's probably the best compromise you're going to get.
Resident Crazy Brit!