Leather is probably the only material the wastelanders can use to make new clothing. Roaming the wastes you'll notice that whatever plant life there is is dead so say goodbye to cotton as an option. Also, we don't know if the wool animals survived so there goes that fabric choice as well. All that leaves is animal hide as an available fabric.
Unless you can scavenge the clothes from Prewar era, you're SOL. Heck, even those aren't in the best of condition anymore. So there you go, leather for everyone.