When the newest Paper Mario game came out, the developers said in some interviews that the reason the games style completely changed in Sticker Star and beyond is that Nintendo got super protective over how their characters are depicted. They weren't allowed to make their own variations of established Mario-world inhabitants (like a professor Goomba or a pirate Bomb-omb or a purple Kamek named Kammy), they either had to use the characters as they were (such as just "Toad", or just "Kamek") or use their own completely original designs. It's also why none of the recurring characters from past games showed up since Super Paper Mario.