It's also useful to keep in mind something Cameron factored into the start of his film: The insurance money. He even has an executive/government representative (can't remember which) point out a huge sum of money which is minus the payload which the ship was carrying.
The reason Weyland-Yutani or any of their competitors (or governments) haven't gone there is either because Dallas turns off the signal (possibly unlikely, since they were still exploring the ship before lowering Kane down and then were too preoccupied with getting him back to safety) or the seismic environmental damage deliberately put in the Jordans scene (keeping in mind that this could have happened at any time, even just a few hours or days after the Nostromo crew had departed). But the reason they hushed up the signal origin would have been mainly because of the insurance money and, of course, fear of criminal prosecution.
The latter actually has subtext in 'Aliens', depending on how you interpret Ripley threatening to prosecute Burke... It could be seen as a sort of analogy between Ripley and the company in general: He's in denial that he did anything knowingly wrong, but she cuts through his self-delusional BS and says she's going to make sure they "nail you right to the wall", forcing him to get desperate.
It's also possible that the warning signal might somehow be deliberately short-ranged. I've often wondered why a government didn't send someone out there, but if it was designed to purposefully degrade after a certain distance, then perhaps it was only picked up by a passing Weyland-Yutani vessel. One which was presumably either unmanned or unable to investigate (otherwise the clause Ash/Dallas mention would have come into effect).
This could possibly also explain why they only sent the Nostromo into the general area, rather than specifically over LV-426, itself: They might not have known the specifics of where the signal was originating. Just that a passing ship registered one being encountered and automatically logged it.
