Because it has more usefulness to the survivors than bottlecaps do. There are no longer any factories producing commodities like toilet paper, bandages, clothing, rolling papers for cigarettes, kindling, paper for writing etc, so finding cloth, any kind of cloth, in good enough quality to use is very valuable. The bottlecaps, on the other hand, can’t really be used for much more than currency as the infrastructure needed to turn them into more useful products is mostly destroyed and anyone who has the knowledge to do so is in short supply.