The '64 Pattern was part of the green combat uniform, to go with the FNC1 and especially to go with the M113 APC. Supposedly a soldier would never have to tire himself out carrying lots of kit because he's never run too far from his APC or 3/4-t truck section vehicle. Twenty-rd mags went into pockets, and ammo in 20-rd boxes in the keepers inside the side pockets, or in tied rubberized cotton bandoleers of that gorgeous DA 63. That's all he'd ever need. Warfare was new and improved.
The story is JADEX (General Jacques Dextraze, hard-as-shrapnel, ex-wartime FMR company commander and later CO, etc etc) was CDS or close to it, took one look at the troops during some exercise and asked the embarrassing questions. That floppy cargo pack was the next best thing procurement could come up with for troop's kit. If my timeline is correct, it wasn't until the 70s when Arctic sovereignty was a big deal that there were proper buys of good winter gear and the US Army jungle rucksack. The one guys still lovingly call the jump ruck. In the 70s the old-timers who knew kept their '51P or the cool kids somehow got British webbing (not US).