They don't need to bring it back---Polygamy is still in the Mormon church. They believe in eternal marriages and after a civil (normal) divorce they won't let women remarry but men can marry again in the church. Same thing if your spouse dies.
Mormons expecting to go to heaven and meet all their wives up there.
Mormons in Utah (a territory at the time) practiced polygamy, but the federal government made ending it a requirement for statehood. So since we're just ignoring federal laws now...
Polygamous marriage was a religious rite in Mormonism, presented as a requirement to get into the best part of Mormon heaven (as an aside, Mormonism divides heaven into 3 main tiers, ranging from "totally cut off from God and everyone you ever loved but still better than life on Earth" to "you get to be God". And this is a simplification since the theology is wild). So when I say "practiced", it's just the usual terminology for when religious people do something dictated by their religion.
Technically, Mormon church leadership has disavowed polygamy, but like other people commented, they still teach that men can have multiple wives in the afterlife, and the current leader of the Mormon church has an... [interesting marriage at the moment (his second wife, so he fully expects 2+ wives after he dies.)] (https://mormondisclosures.blogspot.com/2013/09/same-tract-attractions.html). The exmormon subreddit has more discussion about this, if you're willing to visit Reddit.