...because a change which has happened, and which I'm glad to see, is that some time a while ago it became cooler for students to wear their backpacks two-strapped than to wear it over just one shoulder.
Being a child of Eighties' schools, I did one-strap my backpack a lot, but I would sometimes two-strap it. And feel even dorkier than usual. (This also was the era where I would jerk my head around to simulate watching the world through a shaky camera. I probably looked like I had epilepsy. Or Tourette. Again: awkward kid. You're not surprised.) I got over that due to, as apparently a lot of young people did, the sheer increasing weight of everything we had to carry. For me, it was college; for more recent high school students, a lot of their schools had eliminated lockers, so we often had to carry more.
Then I dated Alicia, who had back issues, and she was especially sensitive to the problems of wearing backpacks one-strapped -- that they were more likely to strain and hurt their backs. (Though the article cites that, apparently, one-strapping doesn't cause scoliosis. But, of course, it doesn't help ease scoliosis, either.)
I now find it profoundly stupid and awkward to wear backpacks one-strapped. Haven't done it in over a decade. I do have a backpack -- a repurposed computer carrying case -- and I've never gone one-strapped with it. I'm sure I look (and am) awkward in other ways. Though now I'm also more likely to have reusable shopping bags and other totes, which have to be worn one-strapped if you carry them over your shoulder. And I remember being impressed to see backpacks that were designed to be slung over one shoulder, but in a way that wouldn't hurt someone's back. Ergonomic!
Meanwhile, watch your backs, people. You need them your whole lives.