You always need an "heir and a spare" and Mary was never in the running to be the Don. Oddly enough in the novel Michael had two sons, and he lost a baby when Apollonia blew up. Mary is an invention of the movies.
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
