The Zerg had, to a certain extent, become weapons of war on the scale of a "war of attrition."
As long as they had enough raw materials, they could churn out troops in a matter of minutes.
As a result, even though the swarm attacking them wasn't the Zerg's main force, it still boasted considerable numbers.
Even a weapon like Genesis was nowhere near enough to clear them out.
The sheer scale of the swarm became fully apparent after the Brain Bug was killed and the Zerg began to lose control and slaughter one another.
Without the Brain Bug as a relay station, the Zerg were effectively cut off from the Overmind. Their "intelligence" talent had never been developed, leaving them with nothing but the instinct to kill.
Across the starry void, Zerg fought one another everywhere. Droplets of blood and severed limbs drifted through space, adding grisly decoration to the scene.
Even so, even with the swarm turning on itself, the pressure it put on the ZAFT-led coalition was immense. It was simply a little easier to handle than the coordinated, deliberate attack before.
The intelligent Brain Bugs understood the threat posed by Genesis and wouldn't let this weapon of war keep attacking them unchecked. That was why they had focused their attacks on the space cities.
Now that the Brain Bug was dead, though, the disorganized swarm didn't care about any of that. Many scattered pockets of Zerg had already bypassed the space-city defenses and were flying off into other parts of space. Some were heading toward the lunar bases, others toward the solar orbits, and some even toward Earth!
"This is looking bad."
"Earth still has plenty of ground-based firepower, but I'm worried the frenzied Zerg might destroy the solar orbital infrastructure."
"Ugh, if the space elevators