1) There were very few fight scenes between the Aliens and the Predators-- in the script, there were supposed to be ten Predators. Anderson reduced it to three.
Anderson lowered it from just five Predators, not ten. And removing the extra two was a mercy for the Predator fans: If the novel's anything to go by, all they did was die. They didn't even really get a chance to fight.
Pred fans weren't happy with one
Predator being killed without a fight - Three would've been overkill
2) No acid proof weapons
It's a test. It's not supposed to be easy.
3) Main reason: Predator's lost...
The Aliens didn't win, either.
I thought it failed because:
1) Too fast-paced. A fast pace is good in some situations, but here it felt rushed. We never really cared for the people.
2) Too much cannon fodder. A whole bunch of people hanging around in the background just waiting to be killed by the creatures didn't sit right with me.
3) Two Predators dying so close together. That just felt cheap.
4) Bad acting and dialogue.
5) Stupid characters.
6) Exposition Boy de Rosa. I know! Let's explain everything
in the last twenty minutes!
7) Too damned short. It only really kicks in half an hour from the end, so people start dying stupidly quickly, and we don't really get any time with the creatures.