While the below is an incredibly juvenile-troll comment made in the original article, ignore the hostile generalizations and he does make a point I believe applies to some. Of course, people will have different reasons why "dude with a gun" appeals to them. I wonder how many the below applies to.
This subject is actually a very good one for discussion, but people are taking it the wrong way. We shouldn't discuss why Irrational make the decision to go with that cover, we should be discussing why a dude with a gun will make the game more appealing to the common man instead of an art-deco image of a flying rhino over a 19th century sky city.
"Most gamers are man-boobed couch fungi who are only able to maintain a scintilla of masculinity by projecting themselves into the lean, heroic tabula rasa of online avatars. The avatars are absurdly hypermasculine. The more ridiculous, excessive, and overblown, the better the sales figures. It's why nerds need to own phones with names like Razor and why their panties need to be branded as UnderArmor."
I want to see that Rhino cover.