Sonic Team’s Takashi Iizuka finally explained why the name change happened.
Over time, after Sonic gained popularity, Sega wanted to unify the names, which was an effort spearheaded by Iizuka.
So, in Sonic Adventure, they decided to combine the two names and make “Dr. Ivo “Eggman” Robotnik” the official name for the mad doctor.
Speaking with Game Informer, he explained that when the original game was localized, the western team, “without consulting” with the team overseas, changed it to “Ivo Robotnik.”