There is no wide and active market for milk bottle tops (or maybe there is one in some remote areas among indigenous people, I don't know, and it is not the matter at hand) and their value in USD terms is unknown. However, there is a market for bitcoins, and its value is known. There are even BTC-based futures, for crying out loud
I must point out that some brokers still do accept bitcoins from their clients as a form of deposit.
So no one knows the exact reasons preventing the majority of brokers from accepting bitcoins.