So thats why starcraft and wc3 use p2p as well? Oh wait, they don't, thats right.
Battle.net started out as pure p2p just like most other RTS games. And it was a mess to begin with. Then, later, they threw servers into the mix so it's sort of a hybrid p2p/server/client system as far as I can tell. Now, I guess Stardock or GPG could have gone to Blizzard and said 'Hey, you've had this system in place for 10 years. It's doing pretty good. Mind if we borrow it for our game?' Would Blizzard have said 'Yes, go ahead, and it's free of charge.' Would Stardock or GPG really even want to do that? Would Demigod even run in the same way that sc and wc3 do on that system? Unlikely is my guess.
Now, take pretty much ANY other RTS game and see what kind of system they have in place. It's p2p.
The only exception I can think of at the moment is World in Conflict. That's because WiC is an RTS where you don't wait to fill up slots before everyone jumps in at the same time. You, as an individual, jump into the game, as it's in progress. Just like a first-person shooter like Team Fortress 2.
Any other RTS you have to fill up slots of human and AI players and they have to all be in sync. Meaning no one is running a different mod on their end that could cause game imbalance or cheating. And then, once the slots are filled, the game is launched.
Can RTS's run in a server/client based configuration? Yes. Why have nearly all RTS's steered away from server/client? My guess is cost but I don't know. I thought I saw Frogboy mention early in the Elemental journals that because of the Demigod fiasco that Elemental may be client/server but don't quote me on that.