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A 3vs3 sweep is fun to watch, a 6vs6 sweep isn't.
By the time Diantha's down to 2 pokémon and Leon still hasn't lost one, you'd be like "okay, you already made a point, Leon is stronger, why continue? we know she can't win"
Imagine if Yamcha vs Yackie Chun was 3 chapters long instead of once. The first chapter already makes it clear that Jackie Chun is superior and Yamcha is absolutely outmatched. The remaining 2 chapters would be pretty much filler.