I feel like he has this presence that immortalizes him as a recurring character, and a worthwile presence too. The man's made himself the main villain of a major sub-series, appears consistently as a boss and playable character in spin-offs. He doesn't need to sing a song as advertisement for a big (and overrated) Mario game to earn my respect.
I mean, didn't spinoff King Boo first appear in Double Dash? A game that was on the same platform as Luigi's Mansion therefore it would've been theoretically simple AF to port his original model?
Either way, glad the true king has finally made his way into Mario Kart. Hopefully this opens the door for the more generic design to start going away and King Boo to look like himself in future games.