Growing up, whenever the topic of Heavenly Mother came up, the standard answer I remember as to why we don't talk much about Her was that Heavenly Father wants to protect Her from the same blasphemies He receives. I guess the reasoning was if we don't know much about Her, then we can't talk about Her and curse Her, too?
Apparently though, some people say there needs to be a "sacred silence" about Her. Even one fictional work took it to the extent "that the Heavenly Mother was so special that God had said we must never, ever talk about her---that He held her on a pedestal where she was never to be seen or spoken to, for fear that her purity would be sullied." I suppose my experience was somewhat similar to that, just not as extreme. I never felt I couldn't talk about Her, I just didn't know enough about Her to say anything constructive.
I appreciated these two quotes that may not provide hard doctrine, but some thoughtful speculation:
Elder John Longden. . . added, "It must be quite an occasion in heaven when our Heavenly Mother bids us a loving farewell for the time being! Perhaps, like earthly mothers, she thinks, 'They are so young, and they might forget [the rules and regulations].'" Longden imagined that before we parted we promised them we would remember. President George F. Richards . . . taught that our heavenly parents are "counting on [us] to honor them, to love them, and obey them. 'Thou shalt honor they father and they mother.'" This commandment applies to both earthly and heavenly parents.
And from Harold B. Lee:
Sometimes we think the whole job is up to us, forgetful that there are loved ones beyond our sight who are thinking about us and our children. We forget that we have a Heavenly Father and a Heavenly Mother who are even more concerned, probably, than our earthly father and mother, and that influences from beyond are constantly working to try to help us when we do all we can.Basically the essay confirms that She's talked about, and there's nothing that says we can't talk about Her. I'd assume, as I said before, that we just don't talk about Her because we don't know enough.