There has been no solid information on why he sold less, he had as many promotion as Meiko, if not more due to the naming contest. Crypton Future Media have always blamed his vocal being that of a males as the reason behind this.
After a famous song brought his vocal back into light he exploded in popularity. Reason being is down to the fact that he was the only mature male vocal (for the Japanese version). Until mid-2008 (when Gakupo was released) he was used to fill roles that required a mature male vocal in songs.