Because you never made a serious mistake in your life? I would trust someone who made a mistake, got burned badly, and learned from it, more than someone who is new to the role. Like, for example, choosing a Conservative leader to be a big-mouth show-off yahoo entrepreneur who has never done a thing in politics, can't speak the language of 1/4 of Canada, has a prior history of donating to the Liberal party, and has said that if he loses he won't stick around to rebuild the party.
And he was. He lost his position as industry minister, he was publicly humiliated, he was out of cabinet for 7 years. But he persevered, he came back, he kept his belief in Libertarian flavoured conservatism the whole time, and now he's back and he sticking to his principles. THAT'S the kind of person you want to lead a political party!