I think the reasons for less active editors is primarily because a large part of common knowledge is already now created and being held in Wikipedia, unlike at the start and first few years of it's existence.

I think generally that as any knowledge base grows, such as Wikimedia, that edits tend to be fewer as general knowledge articles are set in place, and thus remains creating only articles that cover the long tail of remaining knowledge.  Hence, we are in a position in many of the larger language Wikipedia's having less of a need for general article authorship, and instead a need for creating articles that cover the long tails of knowledge with domain experts.

-Thad