Not all academics are bad writers but the exceptions are few and far between. Several recent articles in The Chronicle of Higher Education have attempted to explain why we can't write. There are two types of academic writing. The style you use in your academic papers differs from the style you use in writing for a general audience. There's absolutely no debate about the style of writing in the academic literature: it is horrible and it needs to change.
I want to talk about the other kind of writing; the kind where academics try to explain things to non-academics. I'll concentrate on science writing although I'm sure the same issues apply to history, philosophy, and all the other academic disciplines. I'm particularly sensitive to this problem since I'm working on a book about genomes and junk DNA and it's very different than writing a biochemistry textbook.The latest (Aug. 1, 2016) article is an interview with Steven Pinker, the well-known Harvard psychologist. He's published seven trade books and is widely perceived to be a good example of how academics should write for a general audience [Scholars Talk Writing: Steven Pinker].