Why did Mormons abolish polygamy?

Early members of the Church of Jesus Christ of Latter-day Saints (the "Mormon" church) practiced polygamy because they believed that God had revealed to Joseph Smith that the Biblical practice of polygamy was to be restored. They ended the practice in 1890 after a later prophet, Wilford Woodruff, said he had received a revelation from God to end the practice.

You can read the official answer to this question from the Church, as well as answers from Mormon church members at the "Related Links" below. You can also read the revelations restoring and ending the practice of polygamy in the Church.