I think the problem is more disappointing than mysterious. We can think of numerous causes of it that are plausible. I'm writing an article on another aspect of the problem, on the divide between academic and popular writing. Outside of the academy, the last place people want to feel like they are is back in school. But rigor, clarity, and substance are hallmarks of academic writing, whereas rhetoric, obfuscation, and all manner of fakery and triviality rule vulgar discourse.