Rucksacks have military origins, and they’re built to carry a lot of weight over time and distance. Rucking is the foundation of the United States Army Special Forces training, and rucksacks are built for rucking.
Backpacks, on the other hand, are designed to look fashionable - they feel comfortable when you try them on empty in the store, and they’re mostly associated with school bags or book bags that break at least once a year and end up in a landfill.
A rucksack is not a backpack. No matter what you’re doing, whether it's traveling, training or rucking, if you’re in the military or if you’re commuting on the streets of NYC, your rucksack’s one main job is to hold weight, and do it really, really well.