The Weyland-Yutani Corporation was in contact with Ash on the Nostromo, so they were informed about the derelict ship and its cargo, but apparently the information was filed away and forgotten since they never went back to investigate during the 57 years that Ripley was adrift in her escape pod. The signal that led the Nostromo to LV-426 must have stopped broadcasting during that time, since nobody at the colony knew about the derelict.
It wasn't until after Ripley's rescue that Carter Burke read the coordinates of the derelict ship from her report and sent the colonists to investigate. From wikiquote:
Ripley: Oh, but they will know about it, Burke. From me. Just like they'll know that you were responsible for the deaths of 157
Burke: Wait a second—
Ripley: You sent them to that ship!
Burke: You're wrong!
Ripley: I just checked the colony log, directive dated 6/12/79, signed Burke, Carter J. You sent them out there and you didn't even
warn them! Why didn't you warn them, Burke?
Burke: Okay, look. What if that ship didn't even exist? Did you ever think about that? I didn't know! So now, if I went and made a
major security situation out of it, everybody steps in. Administration
steps in, and there are no exclusive rights for anybody; nobody wins.
So I made a decision, and it was wrong. It was a bad call, Ripley. It
was a bad call.