I suspect because you're writing a tech blog, you sort of somewhat put the blame on the technology but most people cannot communicate well and have never been able to. It's got nothing to do with email or telephone or writing (Aristotle said books would mark the death of civilization) or speaking.

Most folks don't 'know' what they are doing and so cannot teach it to anybody else.
It's the correctly interpreted old saw: Those who can do, those who can't teach.

Most people use this as a slam against teachers instead of for insight into how teaching (explaining a job or task to someone) actually works. You can see this in the music teaching biz easily. The folks who are 'natural' musicians have one heck of a time teaching folks who aren't naturals because the naturals have no techniques for working around common issues that they never had to deal with.

Moms have the same issue. 'Clean your room' seems to them a complete statement but to the husband or child, the room already looks clean. Or picking up the clothes off the floor _is_ clean to them whereas the mom has a different idea of what 'clean' means (often construed as micromanaging when it is really mis-communication or crossed definitions.)