In the case of the early colonies, tabacco was used as money. It had value, it was divisible, and people readily accepted it as a unit of exchange. As tobacco became significantly more plentiful, a grower needed more and more leaves to purchase another product. Eventually, tobacco became so plentiful and the demand for it dropped so low, that tobacco ceased to be a useful medium of exchange. It lost its value.
Money needs to be something that is rare and hence, becomes something of value.
Gold and silver coin serve this purpose, because they are rare, valuable, and easily minted in divisible units. If gold and silver became as plentiful as tobacco or grains of sand, then Au and Ag would cease being valuable. So, money represents a commodity of value that is rare and divisible. Similary, the unit of measure cannot serve as the commodity or item of value. Therefore, money MUST be a substance with equal weights and measures that the public can trust and use as a medium of exchange.