Cigarette butts are too easily destroyed.
That is the real reason why Caps are Currency. They last forever, well, almost forever. Of course we do not see as many as we used to anymore. However back in the day, one would find Bottle Caps all over the place on the ground. These things never seemed to go away. However most caps, as the twist off types were relatively recent introduction, were collected by stores and restaraunts as the bottles were opened by the proprieter's typically, the caps collected and thrown out with the trash, so there was a somewhat limted supply.
So, the original developers of FO had to come up with something that was once common, small enough to carry, is durable, difficult to make without the proper equipment, and is no longer in regular production. Bottle Caps are just about the best thing availible, almost anything else would not meet the criteria.
Of course pre war coins could fit the bill for this too. Not sure as to why that was not used, unless the coinage metal was melted down to make things required for survival.