It's sad to say, but the majority of people cannot follow a logical progression more than a two or three steps. It's not even an issue of "smart" vs "dumb", as many of my quite clever coworkers have this issue. It seems to me to be predicated on the ability to keep a lot of state in your head. If in order to reach an answer you need to synthesize the results of two logical conclusions and you can't keep the first result in your head while you work on the second, you're going to get lost.