Pokémon's Ash is a perpetual loser -- save for some notable exceptions -- and the X&Y anime was no exception. As Ash made his way through Kalos, his lifelong battle against the Pokémon writing staff continued as the series went to greater lengths than ever before in its quest to keep its protagonist in neverending mediocrity. However, this time around, the series had to backpedal a few times because it had accidentally made him too cool.
Touting his strongest team yet, as well as sidestepping Mega Evolution for a more unique Pokémon transformation, the show had little choice but to consistently nerf the aspiring Pokémon Master. In Kalos, Ash regains his signature style and prowess from his Sinnoh days. In peak form once more, Ash and his new team of Pokémon wreak havoc upon the Kalos Gym Leaders. Though he did need two rematches, these defeats were tremendous learning experiences that saw him return to mop the floor with the Gym Leader.
During his time in Kalos, Ash collects his most powerful team of Pokémon to that point in the series. He even had a Goodra, a fully evolved Pseudo-Legendary Pokémon. As the name suggests, Pseudo-Legendary Pokémon are so powerful they are second only to actual Legendary Pokémon. This is where the series begins to realize its mistake and quickly course-correct. Ash catches Goomy, sees it evolve into Sliggoo, then Goodra and then releases it in the span of 16 episodes.
Ash only used Goodra in his Gym match against Clemont, where it emerged victorious and landed Ash his fifth gym badge. When it returned to Ash in the Kalos League, it was inexplicably weaker, only facing two opponents, resulting in a draw and a loss. Leave it to Ash to obtain one of the strongest Pokémon that can feasibly be caught and still be forced to rely on Pikachu for the heavy lifting.
Perhaps the biggest selling point of the sixth generation of Pokémon was the introduction of Mega Evolution. An in-battle, temporary transformation, only a handful of Pokémon species are capable of using it, but the power boost they gain from it is incredible. So, naturally, Mega Evolution was never something Ash would achieve, despite having four Pokémon -- six if you count unevolved Pokémon -- at Professor Oak's lab whose species is capable of Mega Evolution.
Surprisingly, rather than miss out on the in-battle transformation trend altogether, what Ash accomplishes is unique to him and his Greninja. The Pokémon and trainer have the ability to "Bond" with one another, meaning they become one mind on the battlefield. Greninja's appearance even alters to resemble its trainer. This transformation appears to be much more powerful than Mega Evolution as Ash-Greninja, as Bonnie calls it, is able to give the Kalos League Champion's signature Mega Gardevoir some trouble in battle.
In the interest of keeping Ash from getting too strong, this transformation has a severe weakness not found in Mega Evolution. When Ash and Greninja use "Bond," Ash also feels the pain of the attacks that hit Greninja. The result is that Greninja can only maintain this form for as long as Ash can withstand it. Mega Evolution has no time limit or shared damage blowback, it truly has no downside. This flaw in Ash's and Greninja's technique ultimately costs him the Kalos League Championship, as he is unable to endure the grueling battle.
The final nail in Ash's Kalos League coffin was the canonization of Alain, the lead protagonist of the Pokémon Mega Evolution Specials. Alongside his Mega Charizard, Alain wins all eight badges in the time it took Ash just to travel from the seventh gym to the eighth. Alain's sole narrative purpose of defeating Ash was so blatant that the series made it his actual motivation, as he even says he only entered the Kalos League to battle Ash.
Ash's Finals loss to Alain wasn't nearly as frustrating as his Sinnoh Semi-Finals loss to Action Replay Man, because at least Alain had been previously introduced and experienced a character arc before defeating Ash. However, it was yet another example of Ash losing a tournament not because he should've, but because the show demanded it. It's no surprise that Ash won the championship in his next journey, as the show had officially run out of new ways to nerf him and crush our souls.