Perhaps because Snow Leopard doesn't support PowerPC architecture?

From the Apple site:

General requirements

* Mac computer with an Intel processor

* 1GB of memory

* 5GB of available disk space

* DVD drive for installation

etc...

Really? I don't own Demigod but I do have Sins of a Solar Empire. Impulse is only used to download and update the game. I can close Impulse, even uninstall it, and still play Sins online. Did they change it for Demigod?