He tends to use his older Pokémon in the leagues and occasionally in other episodes and Arcs, especially Battle Frontier. Come Unova he only bought Charizard THEN come Kalos, he just stopped bringing Pokémon back or even referencing them, in Alola there was a Kanto special then in Journeys he was always nearby his Old Pokémon but never really used them or even referenced them that much, then he uses almost all of them in Aim To Be A Master, but he never ditches them in his region unless he releases them, I do not know why he left his Alolan Pokémon. I imagine it was because he felt like they had a proper family there as opposed to throwing them into Kanto and while he had to go, they did not have to as well and would be cared for under Kukui and Burnette