Plain, trivial writing is far easier to apply to practice than dense technical writing. It is far easier to talk about. It lets others understand and develop your ideas further. It is empowering.
I suspect the main reason for the lack of brief, plain text is a dominance thing. The presence of dominance hierarchies explains a lot of odd behaviour: why people sometimes get angry at inocuous suggestions, why some people work well together and others constantly grate on each other. If you ask people why they do these things, they will not know. Dominance is not a conscious thing. Few people set out deliberately to disempower others, it just happens by default.
A gift to others, such as a piece of writing placed in the public domain, is a way to assert dominance. One expects the recipients of a gift to be thankful, and to do with it as you suggest. A forbidding chunk of text will achieve this: others will not dare tinker with your design. To write briefly and plainly might make a topic you spent years getting your head around now seem trivial. Once people read what you've written, they will think less of your hard won understanding. They'll use what you wrote in ways you never anticipated, and have no control over.
To write briefly and plainly means accepting a change of role and loss of control. We slip into roles without noticing, and it takes conscious attention to change them.
This applies as much to programming languages as to English.
It's a legitimate criticism of Open Source that much code is hard to comprehed and thus hard to modify. The logical next step in the Open Source revolution is readable source code. That "readable code" is an oxymoron shows how ingrained our habits are.
Simple readable code is empowering; maybe we should call this the Power Source movement. :-)