Bloggers use typographical emphasis effectively to highlight key claims, but claims are rarely key in a legal brief. Blog writing is an exercise in originality of conception, so it’s incumbent on the blogger to draw attention to those original conclusions, whereas legal briefing should seek to minimize the appearance of being original. What warrants emphasis in briefs is argument, not conclusion. Succinct conclusions are easily emphasized. But every part of an argument is equally important objectively, and which part is most important subjectively depends on the reader, so emphasizing part of an argument typographically creates a sense of non sequitur, since the bolded argument doesn't pull its weight. Various parts of the argument are more important for different readers, the emphasized passage or words miscuing them. Typography is too crude a technique for emphasizing parts of an argument, which must display the precise relationships among its parts in nuanced fashion.
Legal writers usefully emphasize headings typographically, but bolded headings must function as headings if they are to avoid the heavy-handedness of bolding parts of arguments. Rather than state part of the following argument, they should summarize or describe the section of text they govern.
There’s also a more speculative reason why bolding body text might always be a bad idea for legal briefs: it may subtly offend the judge by violating a status formality, an informal rule designed to protect the judge’s status. One of the common demands of rules of status formality in courtrooms is that lawyers must always avoid making their own work easier at the expense of making the judge’s work harder. Since it is actually a bit harder to read boldface than roman text, this status formality might apply, the author having other means of emphasis that don’t burden the judge.