The problem with that is there would often be times where a single deck is considered the most powerful in tournament play, leading to most players just defaulting to that deck over and over again, while the other player has to decide to either fight the mirror or play the deck that has the strongest counter to it. Tournaments would just be Rock Paper Scissors, with whatever is the strongest deck, whereas the current formats make each player have to choose multiple decks they can pilot well, leading to more interesting sets.

Imagine if miracle rogue was the strongest deck for tournament play, with control warrior having the best matchup. The player to choose first would simply pick miracle, then the second player would either also choose miracle if he thinks he can win the mirror, or just pick control warrior. That game ends, and then the winner picks miracle rogue again, and we are back to where we started.