Joseph F. Smith tells Eugene England that no revelation exists on premortal explanation for ban.
Joseph Fielding Smith, Interview with Eugene England, ca. 1963, in England, “Are All Alike unto God? Prejudice against Blacks and Women in Popular Mormon Theology,” Sunstone 14 (April 1990): 20
No, you do not have to believe that Negroes are denied the priesthood because of the pre-existence. I have always assumed that because it was what I was taught, and it made sense, but you don’t have to believe it to be in good standing, because it is not definitely stated in the scriptures. And I have received no revelation on the matter.