It was not the main problem, possibly not in the top three, but the basic idea that humans "must" be in the film was definitely a factor. The studio felt that audiences needed humans to connect to the film on a personal level, or some shit.
I would have loved a flick with the Xenomorphs (Aliens) and the Predators running into each other on some far-off planet somewhere, no humans in sight. A well-written screenplay would've given us ways to identify the various players and get to know them as individuals, and it should not be hard to come up with a plausible reason for both species to be there, and a resulting conflict. Aliens vs Predators, just like it says. But it would have been without any dialogue, or at least nothing we would recognize. I think observing and learning how they communicate within their own species would have been cool. No need for subtitles. But I can understand studio suits being scared to effin death by something like that. Way too cerebral, way too "hard sci-fi".
To your point, though, I'm guessing you're thinking that the weak scripts, weak acting, and overall poor writing is was torpedoed the AvP movies. The second one was a bit better, but it had a pretty low bar to clear. I like that they tried, and there might have been one or two interesting ideas there, but both movies were pretty disappointing.