"...his Only Begotten Son, who was in the bosom of the Father, even from the beginning" -D&C 76:13
"The power of God who sitteth upon his throne, who is in the bosom of eternity, who is in the midst of all things." -D&C 88:13
"And now we ask thee, Holy Father, in the name of Jesus Christ, the Son of thy bosom..." -D&C 109:4
"And were it possible that man could number the particles of the earth, yea, millions of earths like this, it would not be a beginning to the number of thy creations; and thy curtains are stretched out still; and yet thou art there, and thy bosom is there..." -Moses 7:30
Et cetera, et cetera...
Also significant are the references to "bowels," evoking connections to the womb:
"Now my brethren, we see that God is mindful of every people, whatsoever land they may be in; yea, he numbereth his people, and his bowels of mercy are over all the earth..." -Alma 26:37
"And thus he shall bring salvation to all those who shall believe on his name; this being the intent of this last sacrifice, to bring about the bowels of mercy..." -Alma 34:15
"Have ye any that are sick among you? Bring them hither. Have ye any that are lame, or blind, or halt, or maimed, or leprous, or that are withered, or that are deaf, or that are afflicted in any manner? Bring them hither and I will heal them, for I have compassion upon you; my bowels are filled with mercy." -3 Nephi 17:7
Also note that the aforementioned scripture with the "mother" earth describes a voice speaking out of the bowels of the earth. Moreover, why not also include the motherly image the pre-descent Jesus Christ uses to describe how he would have gathered Israel?:
"And again, how oft would I have gathered you as a hen gathereth her chickens under her wings, yea, O ye people of the house of Israel, who have fallen; yea, O ye people of the house of Israel, ye that dwell at Jerusalem, as ye that have fallen; yea, how oft would I have gathered you as a hen gathereth her chickens, and ye would not." -3 Nephi 10:5
To make a more daring connection, one could associate the Book of Mormon's frequent references to its being "hid up [in the earth] unto the Lord" with the images of pregnancy and childbearing. Though this may sound odd, it is not without precedent, as the old and deeply powerful Egyptian story of Isis and Osiris demonstrates. To state it very briefly, the god Osiris was destroyed by the god Seth and was severed into fourteen pieces; the goddess Isis then returned to his severed body and used one of the pieces to inseminate herself, after which she gives birth to Osiris anew. Despite the seeming explicitness of this story, it gives a very basic image of the resurrection--namely, that after we are buried in our mother earth, she delivers us anew in a resurrected body. And more importantly, you can associate the symbolic mother's capacity to re-birth a destroyed body with the Book of Mormon's origin story--a record that contains the essence of a people that, though destroyed, lives on again in the book we now hold dear. However, the most relevant takeaway from this principle is the maternal imagery inherent in delivering a book to be reborn out of the ground, making the maternal focus of the Book of Mormon starkly evident.
To wrap up, I think it's very important to realize that, even though we don't really talk about the Mother, that doesn't mean She isn't present in the Church. Even when The Father's and Her images intermingle, we can perhaps discern the immediate presence of a maternal focus in our religion, an emphasis on nurturing, tender love, and growth. To the question of why we don't talk about Her, I can perhaps offer a quote from a famous piece of literature as a response. At the very end of the second part of Goethe's Faust, we encounter a vision of "the Realm of the Mothers," a culmination of the titular character's quest to reconcile himself to the feminine principle. It speaks deeply about the problem, and I exhort those with ears to hear to, well, hear:
"All that must disappear
Is but a parable;
What lay beyond us, here
All is made visible;
Here deeds have understood
Words they were darkened by;
Words they were darkened by;
Draws us on high."