Ridley has said that the Company didn't know about that explicit alien [with a capital 'A'] when they re-routed the Nostromo, just that it may be something juicy.
We can infer that after the Nostromo goes AWOL, they didn't bother messing around with it, for some reason or another [too shady/boardmembers die, whatever, I dunno]. Then Burke catches Ripley's story, sends the colonists to the co-ordinates, then oops lol, then he decides to tag along and make some monies for himself, and a promotion. After Aliens, the Company seem to get clued into what they're dealing with, and chase after it ala Alien 3.
Phew. Conjecture, of course.