Okay, I read your response to /u/zaron5551 and I need to ask a clarificatory question before we go anywhere - when exactly do you think polygamy was a widespread practice that almost all forms of Christianity decided to 'reign in'? Polygamy was not really a feature of the 1st century Graeco-Roman context, and I would say it was not a feature of 1st century Judaism, so why is your question framed around "suppressing polygamy"? Where is your question coming from?

Well, I think your question is a little convoluted, but I think you're asking why polygamy? The basic foundation for it, from a theological perspective, is the polygamy in the old testament. The story I was told as kid is that Joseph Smith asked God why they practiced polygamy in the past and he explained it was part of the 'true' or 'pure' doctrine but through the great apostasy, essentially the time Mormons believe Christ's true doctrine was misinterpreted or morphed by the devil into Catholicism/every church other than Smith's.

For a contextual perspective there were a few other radical religious movements inspired by the second great awakening that had unconventional sexual practices. The shakers are one and there's another I can't remember that practiced free love. So experimentation was kinda in the air.

Looking at Joseph Smith it seems pretty clear, to me at least but I'm an exmormon so take it with a little salt, that polygamy was just Biblical justification for something that already was happening. Since I'm sure this will be a little controversial let go over some examples. The primary one is Fanny Alger a girl Smith apparently had sex with when the church was headquartered in Kirtland, Ohio. If you look at lists of Smith's marriages she's on there but she's the only one for the first few years, like nearly ten, so calling it a marriage seems pretty post hockey to me. Second, people talk about polygamy and thing one man multiple wives, but Smith married women who were already married - often after sending there husbands on missions. Finally, it seems to be common for cult leaders, and at this point Mormonism was definitely a cult - Smith was basically king of Navoou and had his own army - to be sexual predators/take advantage of their incredible influence. I read a book from a jonestown survivor and it described there and you can look at Warren Jeffs and the flds and see a similar pattern.

At first I thought you were asking why they stopped polygamy and there's a simple answer for that. The church was dissolved and their assests were seized plus they wanted Utah to become a state.