Jeans were part of a sailors work uniform in those days, so they were easy to get and screamed NAVY!
Jeans can be hot and they are heavy when full of water but everyone as a really comfy pair of jeans and jeans were used by cowboys and workmen before they were fashionable. 60's and 70's jeans are work pants with plenty of room, not the designer jeans we have today. Jeans offer good protection when waking in heavy jungle/woods etc, if it's good enough for cattle rustling, it's good enough for combat.
About the flip flops, everyone as seen SF pictures from Assghanistan. Some SF troops look like hippies with weapons
Beard, baseball cap, large glasses, sports team shirt....