Probably a lot of them due to an awesome trailer and it being a Aliens game. Also, people trust Gearbox because the Borderlands franchise has gone down so well. Unfortunately, it didn't live up to these things (apparently, because I haven'y played it yet).
I don't know why it would be so freaking hard to make an Aliens like the AvP 2 crossover done by Monolith maybe a decade ago now. That game was unbelievable, so good each individual campaign, each captured what it's like to fight, move, and stalk (or be stalked) like the creature they are with the various abilities. Groundbreaking stuff for an FPS, something we are really lacking in at the moment. And the first decent implementation of the trademark Aliens motion tracker. Every encounter kept you on your toes.