I think a lot has been said on this list over the last few years about a couple of major factors that probably still play a role:

* Shift to mobile device usage and how that affects Wikipedia usage and pageview stats
* Availability of more and more "snippets" in search engine results, which often makes it unnecessary for someone to click through into an article

My own sense is that the prevalence of snippets and smart search results is growing rapidly, I often skip clicking through to Wikipedia if my question is answered by Google.