The 1921 Mercury Dime is a semi key-date in the Mercury Dime series. Only the 1916-D and the 1921-D Mercury Dimes have lower mintages. There are two major factors that contributed to the low mintage of the 1921 Dime.
First, there was a coin recession during that time, which kept the demand low for circulating dimes. Second, the Mint was primarily focused on producing Silver Dollars this year, therefore, producing them in large numbers. This affected the production of some of the smaller coins and the 1921-P Mercury Dime was one of them.