I think that people don't think about things, sometimes, and so fail to reach what might seem like obvious conclusions because they haven't put conscious thought into an area. For instance I don't study complexity, so the first time I read about the concept, I found that very interesting. I had to think it through and read/mentally walk through one or two examples before I was able to "get" it. However now it seems like a given maxim.
I know that if everyone in the world thought the way I did, the world would be a rather different place. So even though to me the way I think seems reasonable, easy, and right, I must conclude that it is not, or it is different enough from the way most people think that the world isn't the way that I'd think it would be. So generally when something seems like it should be common knowledge or a common conclusion but not many people seem to "know" it or "get" it, I am not usually too surprised. I am sure everyone knows or thinks some thing that they think should be clear or obvious to all, but it isn't. It is easy to think the answer is clear when you know what the answer is.