No, I don't mean that the news media are biased politically. I mean that authors are biased by the media they use.
I'm learning about support vector machines (SVMs). There are a lot of books and articles written on SVMs. There are also a whole lot of video lectures on SVMs at videolectures.net (see "kernel methods").
People go into much greater detail in lectures than in text. I like to work with text. I'd like to have a text on SVMs that goes into as much detail as videos on SVMs usually do, and works out the ideas behind the concepts as thoroughly, but no such text exists. For some reason, giving a 5-hour lecture series in which you describe the motivations, applications, and work out the mathematical details is acceptable; but writing a text of the same level of detail, which might take only 2 hours to read, is not.
Perhaps this is because writers are motivated to keep pagecounts low. But pagecount no longer matters with electronic articles. Yet writers still don't want to explain things thoroughly. They certainly aren't saving their readers any time by leaving out intermediate steps. A longer article would take less time to read (and possibly less time to write). Another problem with the pagecount theory is that texts routinely include footnotes and appendices, contributing to the pagecount; yet relegate them to the back of the book, as if embarassed of them, despite the fact that this makes them very difficult to use.
It's especially bad in math, in which writers have a long tradition of deliberately concealing difficult steps and leaving them "as an exercise to the reader". For some reason it is considered bad form to write out all of the steps in a proof, even if adding one or two lines could save the reader five minutes of puzzling. I read an electronic article yesterday where the author said, "These two equations are actually equivalent. Do you see why?"
I think people have adopted a set of cultural biases about what is appropriate in lectures vs. in writing by simply counting observations, without thinking about the systematic sample bias. Speakers speak the way they've seen other speakers speak, without recollecting that most of those speakers were instructors. Technical writers, meanwhile, are picking up their cues from authors of textbooks, which are written with the assumption that a person will be on hand to take you through the details; and applying them in situations where no such person will be available.
Two corollary explanations come to mind. First, writing uses a wider variety of registers and styles than spoken language. Forms and usages that would sound exaggerated or affected in spoken language are socially appropriate in writing. Writing is constructed over time and predominately "for the record," so it uses precise, unforgiving language that suits the specific context of the writing. This is why the first line of a Wikipedia article on some topic in math, poetry, or physics is often indecipherable to a lay reader, even an educated one, without further reading. Spoken language, on the other hand, is first and foremost a form of communication from a speaker to a listener, and is composed and interpreted in real time, even if it's guided by notes. This makes it more fluid and colloquial, and more likely to employ a register that the speaker and listener will both understand readily. Since successful writers use the more precise, ossified language and successful speakers use the more fluid one, they diverge through memetic evolution, as suggested.
The second explanation has more to do with the way writing is taught. I don't know how much it applies to technical writing, maybe somebody can share their experience on that point. Since the Victorian era, prose has embraced brevity. The briefest explanation that still conveys the broad meaning of an author's idea is usually treated as the best stylistically. This sacrifices precision for a kind of clarity, but in a field like mathematics, precision is clarity. Typical admonitions about brevity of style, then, render useless attempts to explain big, scary concepts. Lecturers, however, have the opportunity to pursue digressions and explain minutiae in half-organized ways and still hold the attention of an audience because the lecturer can easily signal the importance of a difficult intermediate step to the wider narrative in a way that would be clunky and perhaps abrupt in writing.