It's always fascinated me that despite being a synthesis of traditional religions and American culture (in the former two cases) these movements have been viewed as alien by most Americans. What has contributed to that, when sects founded elsewhere (Catholic, Presbyterian, Methodist, Baptist) have been successful?
While the sects you mention were in fact founded elsewhere, many of them have existed in America long enough to be considered a synthesis of religion and American culture. The Baptist denomination, for instance, is the most common in America, and the first Baptist church was established in 1638, just 29 years after the movement began in Europe. The U.S.'s Southern Baptist Convention is the largest division in the world. The majority of current denominations' presence in North America predates the U.S. It's really hard to treat them as "foreign" faiths.
Mormonism, however, is a relatively recent off-shoot of American Christianity, one that holds views many Christians consider heretical. A large number of Christians hesitate to call it Christianity. Christianity has been in what is now the US for nearly 500 years, but Mormonism is just over 150 years old. The Christianity that it broke from was already embedded in American culture. Mormonism is definitely gaining more acceptance, but it's no wonder that people didn't think much of a group that broke off from them.