Well, one reason is that the bible is not the most correct book. For example, Genesis 1:1 In the beginning God(Should read Gods) created the the heaven and the earth. The plain and precious things that have disappeared. Also we have 10 words of the original text from the new testament. All the rest is like 100-200 years after the Lords birth. There are an estimated 200,000 translation differences in the new testament alone. So, the fact that there is no mentioning about heavenly mother might be because God the father never brought it up or it was taken out. Even so with Eve's sister wives they might not wanted to have that in there either. I really think that there are a lot more hard doctrine that we are not aware of, just because we are not ready yet as a church or as an individual. I have run into women that told me they received a revelation urging them to live polygamous. I am not saying I buy into that, but who is to say? The Savior also taught certain doctrines that were only for the ears of the inner circle and so did Joseph Smith.
I think that we can safely conclude that our Heavenly Father has at least one plural wife--the mother of Jesus. Anything beyond that would be pure speculation. We can speculate about the length of gestation for spirits in the wombs of celestial mothers and multiple births, but nothing has been revealed about those matters. Mosiah Hancock had a vision wherein he claimed that he was shown that we were all born as sets of twins in the preexistence, male and female, but He was not President of the Church, and his "vision" is not official doctrine of the Church. Thus, we are left to speculate about whether or not we are all children of one Mother or many. It is all pure speculation. I don't know, and I don't have any opinion.