I thought the game was set like a hundred years after Fallout 2. They're already using gold coins in Fallout 2, so why is bottle cap currency in Fallout 3. I never got that.
The only reason that makes any sense, is simply that Fallout 1 used them and it was shoved into FO3 for credibility sake... But IMO it had the opposite effect. In Fallout 1 the central town was called the Hub, and was the HQ for the Water Merchants. Caps were accessible, difficult to make without specialized equipment, and (possibly) backed by Water instead of Gold.
Fallout 2 was 80 years later and had abandoned the use of caps entirely (this was made very clear in the game). The real kicker, is that FO3 is set in DC, and not way out in the middle of nowhere. DC had banks, had a mint, had a thousand bank and corporate money vaults, had gold (and plausibly could have a few bottling companies ~which would be bad in this one instance)... So what other possible reason could there be for adopting bottle caps as the local currency? :shrug:
**Devil's advocate... and its dumb, but... On reflection, one thing springs to mind... Possibly the BOS that left the West Coast, brought caps with them just in case. :shrug:
(I don't know the departure dates from the Lyons part of the tale). Maybe this wouldn't work, but maybe if they thought that use of Caps may have spread Eastward in the years of their common use, maybe they thought that a bag of 10,000 of them might come in handy. :laugh: