The issuance of paper currency is subject to various accounting and quantity restrictions that platinum coinage is not.
31 U.S.C. 5112(k) as originally enacted by Public Law 104-208 in 1996: The Secretary may mint and issue bullion and proof platinum coins in accordance with such specifications, designs, varieties, quantities, denominations, and inscriptions as the Secretary, in the Secretary's discretion, may prescribe from time to time.