Shoulder bags are more cool then lugging a backpack around all the time. But what some girls are anyone that wears a shoulder bag may not realize is that it messes with your posture, backpacks are better in the sense keeps you more balanced. If someone is wearing a shoulder bag, it's good to switch it from shoulder to shoulder every now and then, that way one side isn't getting too much strain. That's what I did, but eventually I ended up not using a shoulder bag or a backpack. I used one of those bags that is like a backpack but with strings that when you pull them the bag closes. It was nice looking to wear, and didn't strain either of my shoulders, plus I was still able to carry everything in it, which was great!
I used to wear a backpack all the time (through high school and university), especially if I was carrying something heavy like books. Now that I'm out of school, I use other types of bags more often, but still use my backpack if I have to carry something heavy.
... I actually don't know why. I guess that's what's in style now. before I moved to my new school I carried my bag around with me all day, but now I drop off my bag at my locker and only carry around my books needed for the classes since I have enough passing time to stop off at my locker between classes, so the bag that I have doesn't need to be in style, but I've just stuck with my shoulder bag from my old school. I don't know why. I have a backpack in my closet. oh well.
I assume you're talking about those in high school in which case it's doubtlessly because it isn't cute enough to wear a backpack, or because they're just generally too f***ing stupid to comprehend anything beyond "hi" and "like omg" and maybe, for the more sharp ones: "oooh shiny"