2000 years - no earth visiting engineers?

Hi all, Great site by the way. I really liked Prometheus, amazing atmosphere and vibe but felt it was a little muddled in places. One thing I didn't get was this: If the doomed group of engineers that the Prometheus crew stumbled on got pissed with earth over 2000 years ago, had a plan to wipe us out - why didn't anymore of them go to earth shortly after their LV-223 accident? There seemed to be 3 more of those bio weapon domes on LV-223, surely not all the engineers were killed? What about the engineer home planet couldn't word have got sent that there was an accident of LV-223 so send some other engineers to wipe out earth? I've got more questions but this'll do for starters! :-) Any thoughts folks? Loving the chat people, but keep it positive, sure the film's got faults but it's still a very good movie, haters go somewhere else!