That's true. Matthew Yglesias is a prime example of an undergraduate whose writings are taken seriously by tenured academic bloggers. Indeed, he is the example used in the article. Oxblog is similarly a good example of a graduate student blog. All of this, however, reinforces the fact that, even online, academics are conscious of academic rank. It was necessary for the Chronicle to point out the apparent levelling of ground among academic ranks in the world of weblogs, even while itself focusing on professorial weblogs in its list at the end of the article. The article finishes with a list of "Blogs of Note." I observe that Matthew Yglesias' did not make the list. Oxblog did, but it is notable in being the only graduate-student authored weblog on the list.
The article also points to two lists of maintained, all-discipline lists of academics: Rhetorica's Professors who Blog and Henry Farrell's. Henry Farrell's stated policy is to include bloggers who are in pursuit of a PhD or equivalent degree, or hold a position at a third-level institution. In other words, graduate students are included in this list. "Professors who Blog," however, by the very nature of its title, is intended to list blogging professors, the senior members of the profession - or, if you're being picky - instructors with contracts to teach at a university of its equivalent. The title is misleading, since at least several of the blogs linked to in the list are authored by people who are not professors, although some have at least taught at a university of its equivalent. Caveat Lector (past graduate school experience; returning to graduate school this fall) and Colin Brayton (current graduate student) are both listed.
I'm not bothered one way or another by the inconsistancies in "Professors who Blog." That's not my point. My point is that, despite the blogging principle that articulateness and knowledge level all playing fields, in academia at least, rank still matters. Tenured, tenure-stream, adjunct, graduate student, undergraduate. It's still an integral part of academic identity.
I'm certainly a part of its perpetration. On the list of Medievalists with Weblogs that I maintain, I identify each weblog by the academic status of its author, if I can discern it.
I'm sure there are other good reasons for professorial weblogs dominating the internet airwaves. The status of graduate student (and undergraduate), while a professional designation, is not (usually) considered a lifelong occupation. (Although PhD certainly has examples to the contrary.) Thus professors might be more likely to present a professionally-oriented weblog to the world. This hypothesized, I can think of several examples just off the top of my head of examples to the contrary (Glosses is the major one). Back to my attempts at generalization.
Another possible reason is experience. One of the hazards and freedoms of being a professor is being solely responsible for the facts and opinions presented in a class. (I've been reading how-to books on academic careers lately. Comments like this reflect that.) Consequently, experience in teaching ought to make one more comfortable with presenting views, opinions, and arguments to the world at large.
Another hypothesis: undergraduate and graduate students running academic weblogs are likely to have had less experience networking with their peers, and thus would be less likely to effectively publicize their work amongst other academic bloggers.
What other reasons are there... hmm.