"Incineroar was chosen because while there’s already a lot of characters who use wrestler-like moves, there was no actual wrestler character in the game. Incineroar’s also the only character they weren’t planning to add from the start. They did plan to add one pokemon from Pokémon Sun and Moon, so they kept one slot open, but didn’t decide on which pokemon until later."

Source: https://www.dualshockers.com/masahiro-sakurai-super-smash-bros-ultimate-famitsu-interview-translation-development-new-characters-esports/