Next time you see somebody carrying a camera backpack or any backpack and unless it's a super heavy pro trekking model, chances are you'll notice something. Most people carry them by one strap. It's possible that people don't like being constricted by both straps or feel more secure if they can keep a hand on it. Or maybe they just think it looks cool. Anyway, bag manufacturers are catching on and most big ranges now have an "asymmetric strap shoulder bag". That's a rucksack with one strap to you and me.
There are some great benefits to this design though. If the bag's built right then you can slide it round your body and access all your gear without ever taking it off. That's a huge benefit if you're worried about security or simply if the ground is wet or muddy and you don't want to put your kit on the floor. Read the whole article here