It's not a rejected design, I think they had planned for Zeba to unmask himself, maybe even be human (similar to Flashman's La Deuce), but I think they ditched that in favor of having him be a mutant who's lying to his officers.
It probably had something to do with the actor, too. The man in the picture is Takeo Shinkai, an actual actor, who played Zeba for the first few episodes of Maskman. After that, he's just replaced by JAC stuntman Hideaki Kusaka. They probably decided to not bother having him unmasked after the switch and the ads with Shinkai, I'm not sure.
Shinkai did pop up in a later episode of Maskman as Kenta/Black Mask's old kung-fu master, though.