Google answer box could be one reason
Many people use Wikipedia to get fast and quick answers. As Google introduced it’s answer box some people might have managed to find what they are looking for without needing to check Wikipedia. (See Why Google Answers failed?)
Many people used Wikipedia to get definitions
Many people used Wikipedia to get fast definitions for the terms they didn’t knew. Because Google’s answer box covered the definitions part well it might be the reason Wikipedia is losing traffic.
Some claim that the Google Algorithm has changed
Some experts claim that a certain change in Google’s algorithms might have devalued some of Wikipedia’s pages. (See Event Summary: Google Core Search Algorithm Update 8th January 2016)
Google might have started favoring brands and their sites over their Wikipedia pages
Roy Hinkis ,SW’s head of SEO, said that Google might have started to rank pages from brands and organizations over the Wikipedia pages that introduce those brands.
CTR might be to blame
The CTR, or click through rate, is believed by the many to be a ranking algorithm. With the introduction of the Google answer box so many people might have refrained from clicking on a Wikipedia’s page even if the answer box grabbed its answer from Wikipedia.
HTTPS might be a part of the reason
According to a research paper published by Wikipedia some HTTPS connections might not be tracked and so they might not be counted.
Google might be pushing their sites above others
According to Business insider Google might be pushing their own sites above other sources even when those sources might be better.
It could be a seasonal drop
Content websites , especially when they are educational, usually see their traffic drop in the period June-September because of the changing behavior in summer.
Google was asked to comment but not responded yet
Google were asked to comment on the Issue and up to the end of august 2015 no reply was given from them.
According to Jimmy Wales the decline is slow but gradual
According to Jimmy Wales, the founder of Wikipedia, the drop was not sudden but it’s slow and gradual. This could give credibility to theories that say that a behavioral change is most likely to be the cause than a sudden algorithm change.
More users access Google with smartphones
Wikipedia is known for its lengthy articles which aren’t suitable for size of smartphones’ screens. As more and more users are using Google with their smartphones they started avoiding Wikipedia.
Why Wikipedia is losing traffic from Google?
Wikipedia’s Traffic from Google Down 11%, Why the Drop? Why Wikipedia is losing traffic from Google?