As Kenshi suggested, the answer's very much the same as why Boo later turned into Pure Boo rather than his Pure Evil self, because of the method of the split. Pure Evil Boo was created from Fat Boo splitting his good and evil sides rather than having all the evil within Boo released (something that would be impossible with how Dai Kaioshin's being suppressed Boo's evil nature heavily), so he'd take far more influence from Boo's current appearance and traits than either of his other two evil incarnations.