U.S. law prohibits portraits of living persons from appearing on Government Securities. Therefore, the portraits on the currency notes are of deceased persons whose places in history the American people know well.
The basic face and back designs of all denominations of the paper currency in circulation today were selected in 1928, although they were modified to improve security against counterfeiting starting in 1996. A committee appointed to study such matters made those choices. The only exception is the reverse design of the one-dollar bill.