Thomas W. Young relays premortal explanation that Church leaders use to justify restriction.
Thomas W. Young, Mormonism: Its Origins, Doctrines and Dangers (Ann Arbor: George Wahr, 1900), 54
Every white adult male is an ordained member of this mystic order. Women, children, and negroes are, of course, excluded. In the beginning in the ante-mundane strife between the Son who proposed to save fallen man from sin, and Lucifer who proposed to save man in sin, a third part of the heavenly host sided with Lucifer and were defeated and expelled. There was another despicable company who were neutral, and because of their indifference were doomed to be negroes during their sojourn on earth. Because of this ante-natal curse the negro is not eligible to the Mormon priesthood.