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flibety
They wanted to conceal their identities, and the cloaks are white because they generally act at night, making it easier to see other klansmen
AnimeBrainly
they wear disguises so that they could continue to be KKK members and are free. The US cracked down on KKK members, and when one was found, they were imprisoned for what they have done. 

Nobody wants to go to prison, and so they hide their identities

hope this helps