i) Despite various disciplinary innovations over the last three decades, we are still asked to become specialists in historically and nationally defined fields, but we are simultaneously told that the essence of literary study is attention to form. Thus, our object of expertise is confused right from the start. Are we formalists or historians? Can we be both?
(ii) Despite the wane of theory, we are still told that literary study must be made "rigorous" through the "application" of various kinds of theory. Unfortunately, each theory or theoretical tradition is taught to us only in partial or fragmentary form, either in "Introduction to Theory" courses or as secondary reading in traditionally (historically, formally) denominated courses. E.g., Let's read a helping of queer theory with our early modern drama! This gives birth to a theoretical "mash-up" culture, in which radically incompatible theories populate our arguments. E.g., I'm a Lacanian postMarxist deeply concerned with a Spinozan debates surrounding postcolonial ethics, especially in relation to the Victorian novel!
(iii) Part of our scholarly training involves reading huge amounts of secondary material larded with jargon. We learn that to be a serious scholar or critic is to speak in a certain idiom. Canny aspiring professionals, we write in the style of what we are asked to read.
(iv) Often, despite our disciplinary self-definition, there is an attendant sense that simply writing about literature or cultural phenomena is not sufficient. If we want the grant or the fellowship that will get us through the next year, we need to concoct elaborate answers to the "so-what" question. We therefore have an incentive to aggrandize the importance of our work: we're being political, challenging norms, overturning conventional modes of thought, etc. Who knew a close reading of a naturalist novel could do so much positive political work!
(v) Finally, after we've written our stylistically mangled dissertations, which try to speak to or satisfy all of the above, we're asked to turn the dissertation into a book that has a "wider audience." Well, we've already written three or four hundred pages in our carefully cultivated "bad" style. We're not likely to make much of a change, and -- I'd suggest -- we've largely internalized the habits of writing that result in the badness of our style. From here on out, this is how we've habituated ourselves to write critical prose. Breaking those habits -- which, if we're lucky, have led to our successful academic careers -- will be very difficult, indeed.
I don't think these are really the origins of bad writing, except, possibly, # iii. The possibility of turning the dissertation into a book is an opportunity to make stylistic revisions. Answering the "so what?" question is also an opportunity. I fail to see why balancing literary history and literary criticism should make us write badly.