The RICO charge is reserved to prove that the money organizations have gained is from crime. Back in the day before the RICO act, you could be the leader of a criminal organization but insulated from the day to day activities, such as bribery, extortion, murder and the courts couldn't pin shit on you.
With the RICO, you don't even have to kill anybody, if someone within your organization did it, you are charged with it.
The RICO act was created in the golden age of the Italian Mob.
Klu Klux Klan isn't a criminal enterprise, it's a hate group that has long lived it's glory days of being the haven for judges and politicians. These days, they are like a club for racist rednecks.
Hate groups aren't illegal.
Plus, we have the Klu Klux Klan Act created in 1871 that was used to reign in the Klan in the 1870s, 1920s and 1950s