The suffering of tzara'as is inflicted only upon the good.

Why?!

Because only the good merit G-d's love, in offering them to cleanse any hidden evil, which hidden evil expresses itself only in one's external faculty of speech, in words, while not evil, yet not carefully measured.

However, those with revealed evil, who do not work diligently to correct and refine their inner-self, are not offered this external reminder from G-d, for how can impure tzara'as be noticeable in impure skin....