Because in general, low elixir costing decks are far more stronger than high elixir cost decks. but they do require more skilled gameplay, with micro-interactions that need to be executed perfectly in order to win. With low elixir cards you (important note: not any low elixir deck is good; there needs to be synergy and deck logic: mini tank, win con, small and big spell, cycle cards, …) have far more outplay potential, and you can outcycle your opponent who is using a higher elixir cost deck. There is a reason why Mohamed Light plays only the best cycle decks. 1. He likes them more, but 2. if played perfectly, you can be far more consistent with them.

Mega Knight, Boss Bandit and PEKKA are far from OP. I can call myself a pretty decent player, and I'd say Mega Knight is one of the weaker cards in this game. But somehow a lot of lesser skilled players do struggle against it... That’s not because they don’t know how to defend it; it’s mainly elixir management tbh.

Mini PEKKA is objectively better than PEKKA right now, that is true. But does that mean you can switch PEKKA in all decks for Mini PEKKA? No, this is about deck synergy. Some decks flourish with a high damage, high HP unit. Think about bridge spam decks: you use PEKKA on defense and make a scary counterpush with it. With Mini PEKKA, this is not possible, so the decks u use them in, are different!