I think part of the problem is not that the audience is stupider than you imagine but that people sometimes use different techniques to learn the same thing. Your explanation may seem obvious to you but confuse everyone else while an alternative explanation that you would have difficulty understanding yourself would be obvious to others.
One example of this would be that some people learn better by concrete examples while others learn better by abstract ones.
I think part of the problem is not that the audience is stupider than you imagine but that people sometimes use different techniques to learn the same thing. Your explanation may seem obvious to you but confuse everyone else while an alternative explanation that you would have difficulty understanding yourself would be obvious to others.
One example of this would be that some people learn better by concrete examples while others learn better by abstract ones.