This was typical of the paper encyclopedias. The initial creation of the Encyclopedia Brittanica was a huge job. The periodic maintenance update was a far smaller one.
I would contend that should be leading to more edits. Few people have the depth of knowledge of expertise to contribute to 'US Foreign Policy' article, but masses can contribute to an article about their home town.
For example, the article on my town doesn't even explain its unusal name. At least half of its residents could make that particular edit... but have no incentive to do so.
What proportion could/would actually cite a source for the edit, though? It's likely not that hard to find a source: the local library probably has some local-history books that explain the source of the name. But in my experience the proportion of people willing to open a book before making an edit is smallish. There are some local "edit-a-thons" trying to improve that, though. I've never been to one myself, but from what I can tell they tend to be organized at libraries or universities, and the organizers have already pulled out some relevant reference materials, so people can more easily start making cited improvements to articles about their local area.
Sociological issues among established Wikipedia editors do contribute do lower retention of desirable new editors, but like Animats I suspect Wikipedia's comprehensive coverage is a significant factor (perhaps more so). It's simply less appealing to do curatorial work than to make large additions to major articles like "Genetics" or "France".
Which factor contributes more to Wikipedia's active editors drop-off, sociological issues or encyclopedic completeness? Has anyone looked at that? I'd be interested to see an analysis of how the trends in the notorious graph at http://commons.wikimedia.org/wiki/File:Enwp_retention_vs_act... correlate with word count and reference count in important articles.