It has nothing to do with right-wing groups and the U.S. government. It has way more to do with the state of Utah, and the fact that almost all the top officials there are LDS. And even though the FLDS and other extremist branches broke off from the mainstream Mormon church decades ago, there's still a hesitancy to go after them because of the entangled history of Mormonism.The U.S. Government has no taste to do a massive investigation of what happens in these communities because right-wing groups get all up in arms about a crackdown on religious groups.
There's also the matter that, for example, in Short Creek (Hildale and Colorado City) all the people in charge that should be protecting children - such as teachers, school officials, police officers, etc. - are almost FLDS themselves, and defer to the prophet. Elissa Jessop Wall, one of Jeffs' victims who fled Short Creek, wrote an interesting book a few years ago about growing up in Short Creek. She knew that no matter what happened, she could never go to a cop, for example. The police force there is FLDS and would just return women and children to their abusive situations because the man is considered the spiritual, moral and financial head of the family.