It is because blizzard wanted the horde to win so they had to intentionally ignore an orbital space ship with a mega lazer and can put teleportation beacons on the ground, allowing land troops to go from point A to point B in a manner of seconds because “it would cause problems”. Blizzard didn’t even think of a half-arsed excuse as to why said space ship was not used. They simply left it out because “we need horde to win”.
This my friends is an example of terrible writing. Something that would plague BFA for its whole lifetime.