They died before arriving on Earth. This can be used as an explanation for why they didn't colonize Earth. Or you can use a Prime Directive like explanation given their reticence to execute criminals (Zod committed high treason and still isn't facing an explicit death penalty, only a de facto one). Or you can use the Prequel comic. It's ultimately immaterial.
Don't forget that General Zod was convicted and sentenced for attempting a coup... that is, betraying, rebelling, and being a traitor to the government he serves.
Those who are traitors have the reasonable tendency to fear betrayal themselves and worry about traitors in their midst.
This is reinforced by the fact Zod only trusted a total of two Kryptonians and himself to go to Earth and gain access to super powers. If Zod absolutely trusted his crew, there's no reason he couldn't have drop-shipped them all around the planet to enforce his plan.
It's clear that Zod doesn't have that absolute trust of his crew. And for good reason. If someone in a lesser caste or rank like, say, Jax-Ur were to taste that god-like Superman-level power... do you see these criminals cooperating with Zod and giving that up out of mere loyalty so that they can serve under his rule?
In a powerless society, Zod's military caste and might can still control things by force. In a powered society, Zod is just another equal. Zod's whole problem with the Council was that they didn't follow his eugenic ideals, so I doubt he wants to give up that control when forming a new Krypton.
