Why do cyclists prefer messenger bags over backpacks? Ok, I know form sometimes follows function - and things can be even more funnily irrational when fashion and trends come into play - but what's the lure of the cyclist's messenger bag?  I see them everywhere, and can't help but imagine them to be a big pain in the lopsided way they hang on a single shoulder, not to mention the strap-over-the-head maneauver required to take one off, becoming even more awkward in tight quarters like say a small cafe.  But there they are, ever popular.  

People who enjoy using them tend to be quite passionate about them - and I doubt it's limited to just the style of the bag. Having little experience riding with backpacks or messenger bags, I ask, why do you choose to ride with one over the other?
I spent the last two years as a backcountry ranger, in Northern California.  There, I wore a backpack almost every day.  Now I am a bicycle commuter in Sacramento and I choose panniers first, a messenger bag second, and the backpack a distant third.  

There are three reasons why the backpack is my last choice in this list.


Any bag I carry ends up being full.  I don't know how that works, but my bag is always full of something.  A full backpack rides higher on my back than a messenger bag.  This raises my center of gravity and throws me off balance more easily.  
It doesn't matter how gently I ride or what the temperature is, by the time I get to work I am soaked with sweat.  With a backpack, I sweat even more because there is less airflow between my back and the bag.  The messenger bag, which rides lower, covers less of my back and I sweat a little less.  Or maybe I just dry out quicker.  
Backpacks with hipbelts and sternum straps are designed to move weight off of one's shoulders and onto the hips and torso.  This works great if you are in an upright, walking position.  When I'm in the drops on my roadbike, a hipbelt is nothing more than a hinderance and a backpack-sternum-strap, though more useful, sometimes tends to choke me.  Weight is again transferred to my shoulders and back, which causes me some degree of pain after a while.  Messenger bags--at least those that are well designed--have a strap which rides diagonnally across the sternum and which functions better in the riding position to transfer weight to the torso.


Panniers are my first choice, though, because they better solve all of these problems.  They are completely off of my back and pretty low on the bike.  For longer rides and for my daily commute, I generally use panniers.  

Not all of my bikes have a rack, though.  I have ridden my bike from Sacramento-to-the Bay Area wearing a messenger bag.  I wouldn't have tried it with a backpack.