Because it's the bag of a hunter, and according to the eminent collector/antiquarian Mon Villegas its origins is in the Philippines. In the '70s, two enterprising Westerners went up to Mountain Province and found this.
Because it reminds you of the best times of your life: high school.
Because if your girlfriend has a knapsack, it wouldn't be so bad to carry her bag around while you're HHWW.
Because you wear it like a man: chest out and ready for battle.
Because you wear it, it doesn't wear you.
Because everything else feels like a purse.