"It's our natural environment, to read things on the Web, to read news stories, and to have something to say," said Ann Althouse, a law professor at the University of Wisconsin who posts her views at althouse.blogspot.com. Compared with spending a year writing a law review article, she said, blogging is fun.
I should add that parts of the process of writing an article are fun, but that there is something un-fun about the long delay between having your ideas and getting them published. I think blogging lawprofs tend to be the ones who enjoy freedom from the editing process. The law classroom itself is very spontaneous and expressive, and if you love talking in class but want a wider, more diverse audience, blogging feels great.
AMPLIFICATION: By "natural environment" I meant text, especially the sort of text that contains mysteries about bias and missing arguments and information. Lawyers can read such things and see what hasn't been said and pose necessary questions and reframe arguments and so forth. We're used to doing that in the ordinary course of our work as we read briefs and court opinions. We just naturally plunge into such things and find material to write about.