Starcraft II Coop: The game inside the game.

[Winthur]

Because they design the tougher Brutations based on perceived player power.
If a couple are significantly more powerful then it either ends with people playing those characters facerolling them, or being near-impossible unless you play those characters.

I don't actually know how they're doing this considering Stukov got his bunkerspam and then immediately afterwards they put out mutations that were insanely brutal towards low DPS commanders (something Stukov is, he wins with endurance and not burst) whereas Zeratul still has yet to be nerfed even though he is consistently the single best pick in all mutations, ever. Even Tychus struggles with low DPS on some objectives and even classic mutation powerhouses like Vorazun, Kerrigan, Abathur or Dehaka have troubles on some mutators (Dehaka and Abathur are both really annoying to play on Just Die! or Polarity and Kerrigan, Vorazun struggle with Black Death.)