Because that is, indeed, the reason why Charizard lost to Brandon's Dusclops. There is no other way around that. It's shown on screen. They were fighting evenly until Ash made that glaring and idiotic mistake. That and his battle against Blaziken were his only losses since returning from the Charicific Valley. Pikachu, on the other hand, has lost many times, though I will admit that he also has a larger body of work. Still, that does not negate that Pikachu's bad outings are far worse than Charizard's bad outings, particularly if you compare post-Charicific Valley Charizard. And remember that Charizard was already pretty strong before hand. In addition, freezing was mentioned in previous posts. Do remember that in Generation 1, the Frozen condition was a defeat in all but name since Pokémon never thawed. That was not the case in later generations and the animé represented that. Pikachu's bad losses and ties, on the other hand, are not because Ash called for a Fighting-type move against as Ghost-type Pokémon when he should already know the outcome. They are because Pikachu's power is at best inconsistent. And remember, Pikachu's terrible showings outnumber his good showings. You're more likely to get the Pikachu that lost to Trip's beginner Snivy than the Pikachu that beat Regice and tied with Latios. On the other hand, Charizard's good showings outnumber his bad showings. And in his worst showing, there is no doubt that trainer error played a significant role in his loss there.