Questions about punctuation may run afoul of some EL&U participants' preferences in several ways.
First and most obviously, punctuation is strictly relevant to writing, not to spoken language—and for at least the past sixty years a central tenet of linguistics is that spoken language is language, while writing is merely a kind of artifact of language. It follows that questions about writing in general and punctuation in particular are not really questions about language αυτος. At best they raise a relatively trivial subclass of issues ancillary to developing a scientific appreciation of how language works, and at worst they constitute an actively harmful distraction or impediment to meaningful inquiry.
Second, because systems of punctuation are matters of convention rather than of grammatical rule, particular punctuation choices ultimately reflect stylistic preferences rather than necessity. But preferences are matters of opinion, and opinion-based questions are (mostly) off-topic at EL&U, so some site participants may downvote punctuation questions in order to discourage people from asking them. For those downvoters, asking whether a compound modifier that appears before a noun should be hyphenated may seem as pointless as asking whether writing should proceed horizontally left-to-right, horizontally right-to-left, vertically top-to-bottom, or vertically bottom-to-top.
Third, some people may downvote questions that bore them—and a lot of people find questions about punctuation and other matters of style boring. Moreover, very few people are knowledgeable about the details of multiple style guides, and acquiring such knowledge is both time consuming and (frequently) frustratingly difficult—factors that may encourage them to be even more dismissive of the whole enterprise. After all, if you think that astrology is just booshwah anyway, why knock yourself out trying to master the minutiae of the pseudoscience?
My opinion is that many style questions are legitimate at English Language & Usage. In my view, questions that inquire into the details, the reach, and the internal logic of widespread conventions (such as those that apply regionally or throughout the English-writing world to various matters of punctuation and spelling) are worthwhile and on-topic to the extent that they seek information about a prevailing real-world practice or understanding. Others here disagree—and some of them downvote.