Possible reasons for low voting
In addition to the low-traffic factor that Neil Fein's answer brings up, the subjectivity of questions and answers may also impact voting. On the one hand, voters may upvote any answer that sounds good (i.e., subjectivity might increase voting); on the other hand, some voters may tend to refrain from upvoting an answer which is insufficiently argued, especially if the answer is not already affirmed by another user.
More objective questions (e.g., questions about grammar or a specific style guide) are probably less broadly and intensely interesting (fewer views and fewer votes).
Because of the frequently subjective nature of questions for Writers.SE and the problem of localization (e.g., critiques are off-topic), as Neil Fein's answer states, good questions are hard to write. Less good questions not only tend to generate less good answers but also detract from interest in the answers.
Possible treatments
Encourage editing existing posts
Making good edits to existing posts, especially to questions, would not only increase the level of activity on the main page (which is probably significant for retaining interest) but also increase the quality of posts.
As a community of writers, editing for clarity, conciseness, and audience (e.g., increasing the "good question/answer" nature as appropriate to a Stack Exchange site) should be things we do well.
If encouraging editing seems especially important, it may be appropriate to develop an ethos that accepts making more substantial changes than other Stack Exchange sites generally accept.
Increase traffic of enfranchised users
Mentioning Writers in comments and chat at other sites — when appropriate — can bring more experienced Stack Exchange users to Writers. For example, English Language & Usage sometimes gets questions that are more appropriate for writers (the reverse may be more common); posting a comment mentioning Writers would not only inform the one asking the question (who may not automatically be enfranchised at Writers via association bonus) but also viewers more generally.
(I answered a hot ELU question and when I mentioned Writers, the one asking the question responded "Thanks, I didn't know about Writers. My question would probably have been even more on topic there than here." One anecdote is not data, but I suspect a number of ELU users that would also be interested in asking questions at Writers is higher than at other sites. Science Fiction and Worldbuilders are also likely sites with likely greater interest in using Writers.)
Asking (and providing good answers to) questions of interest to Stack Exchange users would also be helpful in attracting experienced users. Currently questions on Writers seem to be heavily biased toward fiction. Questions about how to write good Stack Exchange questions might actually be on-topic for Writers (rather than a Meta site)! Questions on technical documentation would seem likely to draw interest from Programmers and Stack Overflow (two significant Stack Exchange sites) and some notification in their chat rooms might be appropriate both to draw attention and answers from experienced writers of technical documentation.
Science writing is another topic that seems less represented on Writers than it could be. Science-oriented Stack Exchange sites are somewhat common.
Questions about blogging, web forums, and even wikis would probably be more attractive to Stack Exchange users than to Web users generally, but these subjects seem to be somewhat underrepresented.
Encourage voting
This is perhaps the most difficult way to increase voting. With relatively low activity, the badges that encourage voting are weaker incentives (not that they are especially strong incentives anyway). Low activity by users (and generally lower enthusiasm) probably correlates with low voting. Those making more posts may be more likely to vote on posts (if only from a sense of doing unto others).
I suspect that improving the quality of posts would also encourage voting more generally. Not only may a decent answer to a great question be more likely to be upvoted and upvoting of such answers encourage relative upvoting of good answers, but there may also be a residual inclination across different questions and a short-term habit of voting. Posts can be improved by good editing and good comments (that motivate editing by the original author) as well as by posting better than existing answers.
