Over on his blog, Stephen Waltz is discussing what often makes academic writing so dreadful:
The first problem is that many academics (and especially younger ones) tend to confuse incomprehensibility with profundity. If they write long and ponderous sentences and throw in lots of jargon, they assume that readers will be dazzled by their erudition and more likely to accept whatever it is they are saying uncritically. Moreover, jargon is a way for professional academics to remind ordinary people that they are part of a guild with specialized knowledge that outsiders lack, and younger scholars often fear that if they don't sound like a professional scholar, then readers won't believe what they are saying no matter how solid their arguments and evidence are.
The second problem is the fear of being wrong. If your prose is clear and your arguments are easy to follow, then readers can figure out what you are saying and they can hold you to account. If you are making forecasts (or if the theory you are advancing has implications for the future), then you will look bad if your predictions are clearly stated and then fail. If your argument has obvious testable implications, others can run the tests and see how well your claims stand up.
But if your prose is muddy and obscure or your arguments are hedged in every conceivable direction, then readers may not be able to figure out what you're really saying and you can always dodge criticism by claiming to have been misunderstood. (Of course, sometimes critics do deliberately misrepresent a scholarly argument, but that's another matter). Bad writing thus becomes a form of academic camouflage designed to shield the author from criticism.
Two salient points, but I would also add that many students are no longer taught English or how to write well. Personally, I had to learn another language before I really encountered English grammar (and I am surely still mucking it up), and my high school classes were more concerned with quote mining and name dropping than effective argumentation. We live in the era of rote learning, where students are rewarded for their memory, often to the detriment of writing skills. It wasn't until University that I realised clarity was more important than shoving in as many quotes and big words as possible.
Waltz assigns The Elements of Style for his students. I too did not encounter that book until University, and it was my Grandpa that recommended it to me. If only more emphasis was placed on such books and at earlier ages, overall writing academic skills might drastically improve.