Why did Mormons stop practicing polygamy? I know that early in the formation of the Church of Jesus Christ of Latter-day Saints, polygamy was common.  It's my understanding that it was an everlasting command from God to Joseph Smith.

Doctrine &amp; Covenants 132


  4 For behold, I reveal unto you a new and an everlasting covenant; and if ye abide not that covenant, then are ye damned; for no one can reject this covenant and be permitted to enter into my glory.


If the covenant is everlasting, why did the Mormons stop practicing polygamy?
If you look further through the chapter, every place where the words "everlasting covenant" are used, it's speaking of marriage in general, and not specifically of plural marriage.  The idea is that marriage itself is an everlasting covenant, not "until death do you part," but for all eternity.

As for why the practice of plural marriage was discontinued, an explanation can be found here, given directly by Wilford Woodruff, the fourth President of the Church, under whose leadership the practice was ended.  

The first part is a formal announcement to the world that the Mormons are done with polygamy; the second part, given some time later, was an explanation for church members of the rationale behind it.  He explains that the Lord revealed to him that political opposition would end up driving the church underground and cause the temples to be closed by force.  This would put an end to the work therein, which the LDS regard as sacred and of great importance.  Also, it would have forced an end to the practice of plural marriage as understood by the LDS anyway, as plural marriages were required to be performed in the temple.  In order to preserve the rest of the work, which the Lord saw as more important than continuing the practice of plural marriage, he instructed the church to stop practicing plural marriage and comply with the laws of the land.