The obvious reason for WayYu's fall.

its all but evidently clear by the second Alien movie that weyland-yutani is strictly a weapons division. Yes they build colonies on outer world only to undermine their true intention at using those colonist as unsuspecting lab rats in there bio-weapons experiments.people ask why in resurrection did weyland-yutani not exist I think the answer is obvious they were at war with the USM United systems military there was supposed to deliver the bio weapon  ( the alien)and they failed. They lost the war with the USM and they took over. Think governmently