Why a 1972 liberty dime has no mint mark?

A 1972 Liberty dime without a mint mark was produced at the Philadelphia Mint, which did not place mint marks on dimes during that year. In contrast, dimes minted in Denver and San Francisco featured their respective mint marks. Collectors often seek out these variations, as the absence of a mint mark indicates a Philadelphia origin, making it part of a distinct category in numismatic collections.