Remember that one of Bungie's flagship titles was an RTS series called "Myth". Bungie before that had made a proto-Halo FPS series called "Marathon".
When they made Halo, they made a pretty good architecture for showing wide landscapes/buildings/vehicles and large-enough numbers of troops onscreen at once. During the alpha experimentation with the game, apparently they seriously took a look at using this to make an RTS (albeit one really outside of the usual starcraft mold), rather than a FPS.
