MRB says HM's name not known because of irreverence.
Melvin R. Brooks, LDS Reference Encyclopedia (Salt Lake City: Bookcraft, 1960), 309-310.
Considering the way man has profaned the name of God, the Father, and His Son, Jesus Christ, is it any wonder that the name of our Mother in Heaven has been withheld, not to mention the fact that the mention of Her is practically nil in scripture?