it's not as simple as that - poisonings, beatings, imprisonment etc have a chilling effect. We have no idea which leaders would step forward if it weren't for the effects of intimidation.

That is to say, someone smart enough to become a political force to be reckoned with is probably smart enough to realise that they will be faced with some terrible consequences for doing so. Their chances of success are extremely small and the chances of punishment is high. The smartest move is not to participate, and so this filters out all but the most principled or naive and foolhardy.