Because a heavily garrisoned desert world will have a much greater signature than the same garrison on a world with a higher dominant terrain multiplier. Your best defense against orbital assault is the enemy being uncertain about how much you've got there in the way of forces, and it being difficult to dislodge you. With this, desert worlds and other worlds with low dominant terrain modifiers and fortification bonuses, ground units are far more likely to be spotted, far more likely to return a large signature and just as important, far more likely to get hit in any subsequent bombardment, be it fire directed or general.
It makes it harder for a defender not to get outmatched in the ground combat, because the attacker holds greater information to the point it likely has an information advantage, while it already has the orbital advantage and a concentration of force advantage potential because he can move more freely than the defender. After all, if the defenders held that advantage they would not be defending this world and be cut off from reinforcements.