I think the reason for Yuvin's decrease in rank is not his selfishness but his attitude and voice tone when he is talking. His voice tone and actions sometimes make him seem kind of insincere and exaggerated at times. Koreans like exaggeration in the form of Kwanghee from Ze:a style and not a more "normal" type of guy exaggeration (how common boys/young adults may act in Korea). His "greediness" for main vocal is nothing compared to other people's greediness for centre. Also, like many other said, he has always been in less popular stages which garner WAY less interest than the popular stages. Over time, people who were voting for him, probably started voting for other contestants.