Well, at the time of the original showing of the film, I suppose it was intended that WY only knew that there was an alien ship issuing a warning. I would think that would probably be enough for them to send an expendable crew down to check it out.
And when the ship they sent didn't ever report back, whoever was in charge shrugged, decided that maybe the warning was somewhat accurate, and buried it to cover his own ass. That's why no one knew about it 57 years later.
Now, though, I suppose the AvP movie would have us believe that at least the Weyland half of WY knew about it all along and had been waiting for a chance to pick one of the beasties up.
And yeah, Ash is obviously trying very hard to do his duty. He warns Parker of the penalties that ignoring the call would incite, he lets Kane on the ship, he's always there with a quick word or a sly suggestion that urges the crew on toward the actions that benefit his instructions. And he never, ever has anything of any real use to contribute to the situation otherwise.
-Pax