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The Kalos League coincided with Pokemon's 20th Anniversary, and saw Ash getting to the finals for the first time. Even before the League there was positive word-of-mouth for XY online, due to production value, story, or Ash being at the top of his game. People who hadn't watched the show in years were suddenly coming back. All eyes were on the anime again, and Ash looked like he was on a hot streak. But they blew it. The sense of betrayal ran deep, and Pokemon's Youtube was getting bombed with dislikes for weeks. The announcement of SM and its whimsical, slice of life direction only rubbed salt in the wound.
After all that, people just stopped caring and moved on. It doesn't help that the SM League itself was chock full of memes and lackluster Battling.