I think this is an attrition issue. Duplicate content in directories gets no search engine love, and Google has specifically said they don't want to show directories that don't provide added information above the snippet, title and url format.
Most submissions that come in are duplicate content. So what this means is it's a little more labour-intensive to run a directory that benefits from search engine traffic, and owners have to give more attention to each listing. The good ones were doing this already, but for the poor ones the game's up.