The problem with math and science content is that there are many possible audiences with many possible backgrounds, and such content tends to require substantial prerequisite knowledge. So when you have a source edited by anyone, you end up with a hodgepodge of different material aimed at different audiences with different expectations of background. It takes a lot of effort and expertise to rewrite this mishmash into a clear and coherent narrative.
This is a much harder problem for an encyclopedia than for a textbook chapter or a journal paper, because each article is supposed to (somewhat) stand alone and be both broadly accessible and somewhat comprehensive. For a textbook chapter you can systematically build up prerequisite knowledge from earlier chapters and you can assume that students will spend significant time and effort working problems and will have some expert guidance and support if they get stuck. For a journal article you can assume readers have deep subject-matter expertise, e.g. have a PhD in the field. In both of those cases you can leave out most information about the topic as clearly out of scope.
Traditional encyclopedias typically punt by just not including much technical detail at all. (Some Wikipedia articles also do this.)
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As a basic example, let’s think about what might be included in an article about “circle”. You can look at this from a kindergartener’s point of view, or a high school geometry student’s, or an ancient astronomer’s, or a physicist’s, or a signal processing engineer’s, or a 19th century projective geometer’s, or a complex analyst’s, or a group theorist’s, or an algebraic geometer’s, or a topologist’s, or a number theorist’s, or an ergodic theorist’s, etc. Some of these audiences are easy enough to satisfy, but to provide deep comprehensive coverage of the way a fundamental concept like the circle is related to every mathematical field is going to take careers worth of background. Which parts to attempt, which parts to skip, and how to organize them is a very challenging set of editorial choices.