Because in RTS gaming, where we have things like Warcraft 3 and such, a game like Halo Wars with such little customization, choice, and strategy involved, just isn't worth the time. It's fun to build up an army of Zerglings for a trap, pull some of your opponents better units along just to have a mass of banelings destroy them. The wonderful glee that comes when, in the confusion of battle, you call in a nuke right ontop of your opponents entire base. Sending in Frost Wyrms to disable your opponents defenses, then one by one knock down your opponents structures.
Know what Halo Wars had? Spawn Tanks. Send tanks to point A. Wait til point A dies or is saved by you. Win game. No strategy at all, it was just a Real Time Game.