What other options did they have? The colonies were rather precious metal poor. None of the colonies were able to produce their own currency and imports of silver or gold coinage was limited.
Colonial copper coinage was issued in the late 1700s.
With no other options, they could declare anything was money, and they kind of did: Continental Currency was printed en masse and was rendered almost worthless, but eventually got redeemed (they paid the militias that were fighting the British with them) for pennies on the dollar.
Once that got settled out, it went back to precious metals. Gold and silver are worth their weight in gold and silver, worldwide.
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