I can see numerous reasons why you'd want the backpack rather than a box on the bike for food delivery (may not apply for other deliveries):
- no need to fuss when you mount/dismount your bike, your backpack is already on your shoulders. Speed is key.
- the focus is not on carrying capacity, it's on speedy delivery. There's only so much food in their backpack.
- because of the time waiting for orders / going to the restaurant / going back to a hotspot, most of the time the backpack is actually empty.
- a box on the back of a bike costs money.