Because, especially in the case of the richer residents of the Mojave Wastland, that would be a lot of weight and bulk to carry around.
I mean where would you even fit over 12,000(enough to buy one of the more expensive weapons some traders sell) bottle caps on you?
Even though you must sell your Pre-War Money, $NCR, Aureus, and Denarius for caps to effectively use them as currency, it would only make sense that people would carry around a combination of the currency types (Pre war money excluded) using caps as change.
With 12,000 caps worth of money that would be most efficiently carried around as 120 Aureus, or 300 $100 NCR notes.
The exchange rate seems to be
$2.5 NCR = 1 Cap
One Denarius = 4 caps
And
one Aureus = 100 caps
I just answered my own question... Oh well I typed too to just delete it all.