I'd guess there have been attempts to do something like this in most fields, and this is one of the few that worked. Why?

One hint may be in http://plato.stanford.edu/info.html#policies

"Contributions to the Encyclopedia are normally solicited by invitation from a member of the Editorial Board. However, qualified potential contributors may send to the Principal Editor or an appropriate member of the Editorial Board a proposal to write on an Encyclopedia topic, along with a curriculum vitae." "By qualified, we mean those persons with accredited Ph.D.s in Philosophy (or a related discipline) who have published refereed works on the topic of the proposed entry..."

Also perhaps http://plato.stanford.edu/info.html#copyright . I don't know how much non-cs academics think about copyright and licensing, but as most accomplished academics are authors, I am guessing it may matter to them that they keep ownership.

"Copyright Notice. Authors contributing an entry or entries to the Stanford Encyclopedia of Philosophy, except as provided herein, retain the copyright to their entry or entries. By contributing an entry or entries, authors grant to the Metaphysics Research Lab at Stanford University an exclusive license to publish their entry or entries..."

This is just one facet of course. I'm sure there are many other factors such as possibly the stature of the Stanford lab, and particular personal relationships and technical skill of those involved, good timing, etc. I'd be very curious to hear from those with personal knowledge of the details.

There is no such thing as "the canonical living expert" in most philosophical fields, since the most important figures in any given field tend to disagree with one another very strongly. You could call them experts in their own positions, but they are hardly the ones that you could rely on to deliver a balanced introduction to the field as a whole.

Most SEP articles seem to have been written by younger professors (mostly in their 40s) whose daily job is to teach balanced, comprehensive, introductory courses on general topics. They might not be world-class experts in any single field, but they have broad knowledge and teaching experience. They might have edited anthologies or organized conferences on the topic. They know what points students are prone to miss. They know which theories are frequently misunderstood. This breadth, not depth, makes them ideal authors of encyclopedia articles.

Don't know about Dr. Orend or the philosophy of war, but the authors are typically quite well regarded philosophers, if not necessarily the world expert on X (if people can even agree on that).

That said, I think writing an SEP article is a nice line item on a resume but not the sort of thing that makes your career. I think the authors typically write the articles after receiving some recognition.

See also:

http://blog.scholarpedia.org/2013/11/04/wikifying-scholarly-...

My interpretation has always been that philosophy students and professors just really like to share and discuss ideas as an ends unto itself, with little regard for self-interest.