KHS says we don't talk about Heavenly Mother because of how we would slander Her.
Kathryn H. Shirts, “Women in the Image of the Son: Being Female and Being Like Christ,” in Women Steadfast in Christ: Talks Selected from the 1991 Women’s Conference Co-sponsored by Brigham Young University and the Relief Society, ed. Dawn Hall Anderson and Marie Cornwall (Salt Lake City: Deseret Book, 1992), 95.
"If we have a Mother in Heaven, how come we never hear about her?" The teacher’s reply was that God was protecting her name from the kinds of slander that human beings direct toward the names of the Father and the Son.It was a clever reply, and, at the time, we all thought it was quite satisfying. None of us realized then that this answer described a lady not quite up to taking care of herself in a tough world, an image drawn purely from certain human conventions and not from divine reality.