Like, because you'd be all bored. With nothing to do.
I didn't regularly watch the series on which Serenity is based, which is rather like telling you I didn't regularly sprint home from work, frantic and out of breath, in order to apply sandpaper to my eyeballs. And before you crucify me, understand that I didn't know anything at all about the series. All I knew was what Wikipedia told me: its genres were "Comedy-drama" and "Space Western," and these ominous combinations filled me with dread. So I didn't expect much from the film; maybe something like Brisco County Jr., but without Bruce Campbell, and viewed in the hard, cold light of my mid-20s instead of my much more accommodating low teens. Basically I wasn't looking forward to it.
I'm also not much of a sci-fi person. I love the occasional project like Blade Runner or Gattaca, but I'm the guy who groans audibly during Star Trek episodes and gets shushed by everyone else in the room. Then they proceed to call me an uncultured slob, question my grasp of string theory, and invoke something called "The Best Of Both Worlds," and in time I retreat bewildered to my stash of Wild Turkey, bitter tears stinging my eyes.
So I wasn't sure how much I was going to approve of Serenity, and indeed at first it wasn't much. Over the course of the film, though, it becomes clear that these guys--most of whom I don't know from Adam--are actually trying to do something unique and authentic, and I'm sure a lot of that can be attributed to the series everyone is so damned in love with. The Old West motif persists admirably throughout the film, from the language to the weaponry, and even if everything else were terrible I'd have appreciated it for that creativity alone.
But everything else isn't terrible; it's actually pretty good. Characters are developed, conventions are defied, and some well-directed scenes ensue. This isn't Citizen Kane, but it's The Sixth Sense or The Fifth Element--a film that, operating within its genre, does what it's supposed to do very well.
And really, this is an aspect of genre criticism that should factor into the reactions of film critics more often than it does. Universal concepts like characterization and pacing are important--and more basic mechanics like editing are even more so--but critics too often dismiss a film for adhering to basic genre conventions. Oh, Way of the Gun was violent? No shit, guys! It's a sardonic action movie! This is what it does!
What gets me is that I'm guilty of the same thing. We all are. Plenty of people find a way to like both Magnolia and Aliens, but those same people--myself included--cannot fucking tolerate a casually-paced romance. If there's a genre film to which you're viscerally opposed, it's really hard to suppress your "this is awful" reflex, even if it hits all the notes it's supposed to. It's like saying Shakespeare's a hack because you only read mysteries where the lady's cat solves everything.
The point of this blathering diatribe is that I don't particularly like the genre Serenity belongs to, but I wholeheartedly acknowledge that it does what it aims to do pretty damn well. It's a little like the first Star Wars film in its ability to introduce a new universe and to completely avoid being derivative. If it weren't for the mythical quality and unassailable cultural importance of Star Wars, I'd compare it to Serenity more readily.