Pikachu has been with Ash through thick and thin since the beginning of the Pokémon anime. While the two did not get along very well at first due to Ash's initial lack of knowledge on how to be responsible for his Pokémon, they were able to make up and move past it. Although Ash would assemble a new team of Pokémon upon traveling to a new region, Pikachu always accompanied him, no matter what.
After training and battling with Ash for so long, it's not unreasonable for one to think Pikachu would eventually become nearly impossible to defeat. Yet whenever Pikachu enters a new region, all its strength is suddenly diminished; it even ends up getting defeated by weak Pokémon under trainers who have just started their careers. Although fans believe this is done to keep the anime from becoming boring, there may be in-universe reasons for why this continues to happen.
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Pikachu Was Sick at the Beginning of Ash's Hoenn Journey
While anime fans like to see arcs revolving around weak or inexperienced characters becoming stronger and slowly rising to the top, they also don't like seeing too many moments of that same character becoming overpowered. Without Ash or Pikachu experiencing a loss or having something new to learn, the Pokémon anime definitely would've started to feel repetitive and boring, making their journey to become the very best seem like a piece of cake.
To combat this, there are plenty of in-universe incidents that appear to have taken a serious toll on the electric mouse's overall strength. In the Season 5 finale "Hoenn Alone," Ash is seen traveling to the aforementioned region via boat when Pikachu is suddenly captured by Team Rocket. As Pikachu is restrained to the magnet, the trio does not hesitate to activate it, which in turn soaks up all Pikachu's power and makes it seriously ill. With an electric type like Pikachu coming into contact with an electromagnet, it should be no surprise that the consequences wouldn't end well.
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Ash's Pikachu Was Struck by an Intense Electrical Attack
Ash and Pikachu appear to have an unlucky streak of getting seriously injured at the start of a new region. The duo make their debut in the Unova region in Season 14, Episode 1, "In the Shadow of Zekrom." Upon arrival, they are greeted by the presence of a strange black cloud in the sky, with blue lightning bolts zapping out of it. Caught up in the chaos, Pikachu ends up getting hit by one of these strikes, revealed to have been caused by the titular electric legendary Pokémon.
This intense loss of power is seemingly later confirmed when Pikachu is defeated by a weak-looking Snivy. Although the mechanics of Pokémon getting stronger don't work in the anime the way they do in the games, fans have often chastised Ash for this oddly impossible loss.
While fans were annoyed at this particular incident, it could be that Zekrom's intense power was a large contributing factor for Ash and Pikachu having to go through many things all over again, strength and knowledge-wise. Although the events with Team Rocket and Zekrom may seem like mere plot devices, both have kept Ash and Pikachu's Pokémon journey engaging and full of hardship.
