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One of the Rangers who had been killed was the Lone Ranger's brother, Tonto made a mask from his shirt for the Ranger to wear until they caught the gang (so that he wouldn't be recognised by Cavendish). After they caught the gang the ranger decided to keep wearing the mask until the West was free of crime. So, that is why the Lone Ranger wore a mask.
There was an interview, in which Kishimito was asked the same question. His reply was that he saw ninjas as being mysterious, so, he made Kakashi wear a mask, but he didn't do that with other characters because it's too difficult to get their expressions down without the entire face. Maybe it has something related to ANBU. Kakashi was an ANBU, and all the members of ANBU wear masks.