Why? Because it is not slang, the etymology of which this word is historical.
It originates from the offer in historical times of "detachable coupon bonds".
It has come to be applied to white paper.
The original "detachable coupon bonds" may have been white, but actually could not always have been so.
Many are very beautifully engraved and have been exquisitely designed in a display of the most splendid colors.
Mostly these bonds have been redeemed, but not always.