They're not leading men, they can't carry a movie and hope to sell tickets.
Michael Biehn isn't a leading man?
The Terminator? Hello? The dude had to carry that film on his shoulders. Not only did he have to deliver exposition, he had to deliver it backed by a lot of energy and try to pack in some kind of nuance. Almost all of the plot sits in his lap. Not to mention he's pretty much been a consistent performer and actor his entire career. Tombstone, Navy SEALs, The Abyss, Aliens, Planet Terror, Blood Dragon.
And Lance Hendrickson may not be a "leading man" but he is one helluva strong supporting actor and a worker. That guy put's it all in for his craft. Even people who dislike AVP 04, and the 2010 AVP game think Lance was a highlight at least.
Weaver didn't start as a draw. She was literally no one when they made Alien, and to be perfectly honest if the behind the scenes situation is anything to go by it could have very easily gone the other way for Weaver.
If ALIEN III went ahead in 1989 or 1990 with Lance and Biehn, it would have done just fine.
Wasn't the "no guns" thing one of the demands she made of Alien 3?
She wanted one alien, no guns, and to die. She also wanted to have sex with the Alien since Aliens. She wanted the death scene too, but Cameron said no to that.