I'm amazed people forget how controversial this was. Hell, the official Japanese YouTube channel removed likes and dislikes and disabled comments specifically because the videos related to this episode got dislike-bombed.
Also as others have mentioned there was a lot of hype before this episode. Hell even the Japanese title baited people in (I checked and the title for previous leagues' final episodes were usually along the lines of 'X League conclusion' or 'Last battle').
Additionally, as
>>53119933 said, it's hard not to argue that Ash winning the Alola League (granted, it was a joke league) and eventually the PWC as a direct reaction to fan outrage about the Kalos League.