Interesting, to be honest I was expecting the thesis to be along the lines of "people cannot write because they cannot think clearly". It seems to be that with so many distractions and information overload, the science and art of thinking and articulating ideas clearly are considered as less important.
Edit: I also feel (highlighting "feel", since I don't have data to back this statement) that people tend to read less structured and long content than before (as opposed to tweet-like content and short click-bait articles). Not sure how this influences how society thinks and writes.
Whatever the reason for why people can't write, I agree it's a big problem.
I've found from experience that if I write to colleagues using jargon that they don't all necessarily understand what I'm saying or some accuse me of omitting essential detail. When I try to remedy the problem and rewrite using simpler terms then I'm accused of stating the damn obvious and or being prolix.
It seems to me that there is no satisfactory answer, that is unless one has the good fortune to be a member of that very small and gifted group who can write both succinctly and clearly.
TLDR: Explanatory footnotes (or links to Wikipedia) are awesome.
In print also consider insisting on indices, glossaries, tables of contents, clarity of page numbering, standard numeric preface page numbering, explaining all foreign language, and always including a map.
Edit: I also feel (highlighting "feel", since I don't have data to back this statement) that people tend to read less structured and long content than before (as opposed to tweet-like content and short click-bait articles). Not sure how this influences how society thinks and writes.