Would you prefer they made Halo for the Wii? Because I think that game already sells pretty well on another system.
The idea is to differentiate yourself from your competitors, not just repackage something popular. Other games will come, for all 3 systems. Don’t bail on a system because they figured out how to appeal to a targeted market before they start filling in the voids already swamped by another (or 2 other) systems.