>>53478485There was thematic importance in his first League loss, how he was still inexperienced as a Trainer, how his Charizard refused to obey him in a crucial moment, how he had squeezed through a lot of situations through pure luck rather than skill. They gave him the win with the Orange Islands, but I don't think anybody would take a filler region seriously. Similarly, I don't think people would be satisfied until Ash won a League so the Battle Frontier wouldn't be a good place to end it either. While Johto was mostly filler, there was the narrative loose end of his rivalry with Gary. He was able to win that even if he lost the League. But from there he loses because the show is designed to run in perpetuity.
Hoenn and Unova are throwaway Leagues, they only exist for him to lose. Sinnoh was taken more seriously, and the gap between DP and BW was the most notable soft reset in the anime at the time, so the Sinnoh League does feel like its own conclusion to the previous narrative. They let him get to the semi-finals but left viewers thinking he would have been good enough for the finals, he was taking down Legendary Pokémon and a deus ex machina Trainer with a team of Legendaries was the only thing that could stop him. Unova tried to do a back to basics inexperienced thing, which wasn't popular. Kalos went more shonen and built him up but couldn't allow him to win because his win was designated for the 20th anniversary Generation. Alola was mostly SoL, but the game narrative says the protagonist is the first Champion, so it allowed the same even if the League felt like a smaller event. From there, the final series tried to be a world tour with a set final milestone with the vague Champion tournament that had only been alluded to.
Part of the reason to end it now came with the general decline of TV viewership. The anime is a marketing tool for children, and if it's not needed, it's just wasting money.
