My understanding is that the main line church came to the realization that it wasn’t an insular frontier church and de facto government anymore around the end of the 19th century. There wasn’t a mob of Protestants looking to wipe them out like in the early years and they couldn’t sustain their communities without outside converts and abandoning self-destructive social norms.Yeah, killing Mormons to Joe US Newsconsumer equates to doing a driveby on the 19year old "elders" that come a-knocking; we're probably more in the territory of Mountain Meadows Massacre. Mormons are hardcore and even if the LDS in Utah proper is relatively mellowed, FLDS branches are anything but. See also Blood atonement; wikipedia on Mormonism and violence (yes, Wikipedia is syphilitic but at least it's a jumping off point for further resarch)
They chalk it up to the Lord revealing a better way, but it likely had a lot to do with wanting a fighting chance at winning converts in a world that was rapidly evolving from industrialization and changing morals. Certain traditions associated with the church usually didn’t play well in the age of the printing press, and definitely wouldn’t in the age of the telegraph.
So to help spread the word they decided to drop the whole “plural marriage” and “armed secret enforcers” and “using capital punishment for weird blood rituals” thing. The FLDS then said nah we’re good we kinda like that stuff and the mainstream media has been confusing the two to this day. The LDS insist that God told them to knock it off, and the FLDS decided not to acknowledge the church authority anymore.
I’m not as familiar with the Church’s settlements in the West and Mexico, but my understanding is that by and large they got in line, with a few nutty exceptions.