The traditional designation of an autobiographical account as a literary genre has always gone by the name memoirs, in the plural, not the singular. Recently, however, writers and readers have begun referring to it exclusively in the singular, viz. memoir. This change can in part be accounted for by the fact that of all contemporary literary genres memoirs was the only one whose designation occurred in the plural only, the singular being reserved for other kinds of written account such as a memorandum, notice, special study, monograph, or history. The change, therefore, can be seen as a lexical normalization.
A possibly covert other reason for the change is the simple fact that speakers and writers––particularly of American English––are ignorant of the original meaning of the word, namely ‘remembrances’ or ‘memories’. The (French) form that took root in English to mean the genre obscures its origin and its attendant meaning, hence facilitating the new recurrence to the singular and the oblivion of the traditional plural.