We know there was a crew of Engineers which was supposed to wipe out the Earth and it was stopped by something before doing so. But what still bugs me, why another Engineers did not finish the task? One would say they have more than one ship or military base...
Even if the other Engineers didn't know about the fail immediately, it was ~2000 years ago, surely during that time they would figure out something bad has happend...
It seems fairly clear to me that the Engineers don't follow up or at least don't follow up on a timescale remotely close to what we are familiar with. After all, if they did, they would have come back to the planet on Prometheus and helped out that group of Engineers.
So it seems that the group of Engineers in Prometheus were tasked with wiping out Earth. They got wiped out instead. It seems obvious that the Engineers populate the galaxy and the galaxy is quite large, so this could contribute to why they wouldn't interfere with each other's tasks. In other words, the group of Engineers on the Prometheus planet attempt to seed X number of planets - one being Earth. Other groups of Engineers not on the Prometheus planet wouldn't necessarily know which planets that particular group seeded. What would the purpose be? It would take millions of years for the planet to evolve sentient life.
So either they don't follow up with one another or it just takes a really long time for them to follow up.
Ask yourself, what would happen if a government funded project to, say, eradicate a parasitic fungus from the jungles of some far away country, resulted in the catastrophic death of everyone involved in the project, and the need to quarantine the entire facility?
That project would probably be cancelled and public support for it would plummet, to the point that proposing a new attempt might even be taboo.
Now, as yourself, what if said project was a secret undertaking, and it was not possible to even admit that the project had failed catastrophically?
In any case, we don't really know anything about Engineer society, about their government, ideals or social dynamics, but it stands to reason that losing your entire research facility and crew might have an impact on their future plans.
As far as we know, maybe the facility in the movie was the only cache of black goo they had... Which, seeing how dangerous the thing can be, would make sense.
