Rav Ozer Alport addresses this question in his weekly parsha sheet.
"The Shaarei Simcha answers that people who are well-known for their evil ways will have a difficult time trying to ensnare innocent, unsuspecting victims. On the other hand, those people whose wickedness is concealed represent a much greater threat. One of the purposes of tzara'as is to separate the gossipmonger from society to protect them from his negative influence. However, if somebody's entire body is covered with tzara'as, it will be plain to all that he is to be avoided, and there is therefore no need to rule him impure and distance him from others. Alternatively, Rav Reuven Zelig Bengis explains that the purpose of tzara'as is to tell a person that he has sinned and to motivate him to approach the Kohen who will teach him how to examine and correct his ways. If he doesn't do so and the tzara'as continues to spread, this proves to him that he still has work to do. If, however, the tzara'as has already spread and covered his entire body before he even approaches the Kohen, he will become despondent if he is ruled impure, thereby defeating the purpose. As a result, the Torah rules that he is pure."