

Here is the short answer: because only
certain marriages are recorded in Heaven.

Now for the long answer:

In Doctrine and Covenants section 132, the 
chapter discussing eternal marriage (and, yes,
plural marriage), the distinction between
sealings under the priesthood and other 
marriages is revealed.  

When "the children of this world marry, or are
given in marriage" when they receive "the 
resurrection from the dead, neither marry, nor are
given in marriage" (Luke 20:34-35).
Jesus was simply teaching that marriages "until
death do you part" are not in force after death.

However, the Doctrine and Covenants continues 
describing eternal marriage.

D&C 132:19
   And again, verily I say unto you, if a 
man marry a wife by my word, which is my law, and
by the new and everlasting covenant, and it is
sealed unto them by the Holy Spirit of promise,
by him who is anointed this power and the keys 
of this priesthood; ... [ shortened for brevity AI]
and shall be of full force when they are out of
the world; and they shall pass by the angels, 
and the gods, which are set there, to their 
exhaltation and glory in all things, as hath been 
sealed upon their heads, which glory shall be a 
fulness and a continuation of the seeds forever
and ever.

The Lord told Peter "whatsoever thou shalt bind
on earth shall be bound in heaven" (Matt 16:19).
Do you doubt that Peter was given the power to 
perform sealings?
Peter thought so because he taught that husbands
and wives were "heirs *together* of the grace of
life" (1 Peter 3:7).

"In order to obtain the highest" (degree of
celestial glory), a man must enter into this
order of the priesthood" (D&C 131:2).
When a man and wife are sealed they truly become 
"one flesh" because their eternal "increase" 
(destinies) are enjoined completely.

Our Father has an eternal companion (and maybe
more because of the plural marriage conditions
of the law) who participated in our creation
and is equally concerned with our progress here.

There is no scriptural basis for this doctrine.
If fact, the only mention of our Mother is in
one verse of a hymn written early in the history
of the Church:

    O My Father

    I had learned to call thee Father,
    Through thy Spirit from on high,
    But, until the key of knowledge
    Was restored, I knew not why.
    In the heav'ns are parents single?
    No, the thought makes reason stare!
    Truth is reason; truth eternal
    Tells me I've a mother there.

Why don't we hear more about our Mother?

1.  Because our Father presides under Priesthood
authority (which is not a calling for Her);

2.  Because we don't all (necessarily) have the
same Mother it would be confusing for worship;

3.  Because our Father wishes to withhold Her
name and titles because of how some people
degrade sacred things.