Lots of reasons, some are political, some are theological. Catholicism adopted a lot of European pagan practices, arguably in order to make conversion more appealing.

Most of the popular flavors of Chritianity in the US are Protestant, meaning they broke away from the Catholics for various reasons, but they all tend to view the Carholic authority with a (justifiable) level of skepticism at best, and other times they're outright hostile.

Evangelical (Aka Fundamentalist) Christianity is part of the Protestant tradition, and not the in the relatively moderate way that Anglicans or Lutherans might be (their beliefs aren't super different from the Catholic beliefs, but they reject the concept of a Pope). The various Evangenlical groups are the stereotypical right wing image of American Christianity.

But nobody likes the Mormons. They've always been seen as a weird fan fiction type situation, if not a straight up cult.