[tshaddox]

I think it's because pretty much any conceivable game design decision will only be a difficult decision to make if there is disagreement on what is fun. In other words, you'll never have two game designers trying to make a difficult decision where one designer says "the thing I want to do is less fun" and the other designer says "the thing I want to do is more fun." They'll both think that the thing they want to do is more fun, because in the context of game design "fun" is essentially synonymous with "good."