So many normal countries (like Canada and Germany) ban hate groups. Why hasn't the U.S. banned them?
Who decides what's a 'hate group'?
This is the answer. The FBI tracks violent/potentially violent groups like the KKK and various separatist militias in the west. But there's no government agency that looks at, say, the Westboro Baptist Church.
Allowing the government to designate hate groups opens up a can of worms. Could a Democratic president decide Focus on the Family is a hate group? Could a Republican president buy into right-wing nonsense about Black Lives Matter being a radical organization?