The film's official novelisation offers some insight into why the Kryptonian colonies failed. The major problem seems to be that no matter how fabulously wealthy they were, these colonies lacked self-sufficiency and seem to have relied on Krypton to continuously provide them with supplies. Over time, the Kryptonian civilisation waned and their colonies were cut off, drifting into disrepair and finally abandoned entirely:
“What happened?” he asked.
“Space exploration was abandoned,” Jor-El explained. “We exhausted our natural resources. As a result, our planet’s core became unstable. Eventually, our military leader — General Zod — attempted a coup. But by then, it was too late.”
Worse still, when the colonies were cut off by Krypton's decision to withdraw from space travel many colonists were evidently left to die, lacking the ability to return
All they found was death. The skeletons of long-dead colonists
littered the crumbling structures, which were being eaten away by the
relentless winds. Cut off from Krypton after space exploration became
a discarded luxury, the abandoned outpost had withered and died,
perhaps even before Krypton had. Zod and his followers found no
long-lost brothers and sisters.
As to why the tech is still lying around, it seems that it was left unused by the failing colonies in the faint hope that that they could use it when times improved.
Yet the expedition still yielded some benefits. Work crews from the
Black Zero salvaged everything they could from the dead outpost—armor,
weapons, even a massive World Engine only somewhat smaller than the
Black Zero. The towering mechanism had apparently been left idle after
the colonists lost hope,