I had to think about this for a minute. I think there are two types of comparison. If we are comparing things that are similar, but to different degrees we need AS twice.: Tyreek Hill is not as fast as Usain Bolt. (they are both fast; one is faster) But if things are complete opposites, only one AS is needed: Beer is not sweet, as Coca-cola is. (It's not that beer is not AS sweet AS coca cola; beer is not sweet at all). That's my understanding of it at least.