The Comfortable, Old Shoe of Negation

Unexplicable and explainable are used almost on par with each other in professionally edited prose. But interestingly, unexplainable is used about twice as often as the positive explainable. And so what we find is, for whatever reason, engl speakers really like this negative formation of words. It's far and away, in terms of published prose, the more common form. Maybe, is that it's hard to understand?