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.
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.