I've been pointed towards this recent Tweet from Lead Geo Developer Advocate, Mano Marks, that explains that the Wikipedia layer was removed from Google Maps because of low usage.
@cageyjames @tooshel They were turned off due to low usage. However, they are present in Google Earth.
— ManoMarks (@ManoMarks) August 20, 2013
Google is continuing to remove useful functionality from Google Maps. The new look Google Maps has no terrain layer, 45° degree imagery or weather layer. Now Google has removed the Wikipedia layer from the classic Google Maps.
The Wikipedia layer on Google Maps was one of the most useful functions. When exploring the world on Google Maps and you come across a strange building, structure or land-form it makes so much sense to be able to switch on a Wikipedia layer and discover what you are looking at.
A Wikipedia layer on Google Maps is such a good fit that it is very hard to understand why Google would remove it. I can think of one reason - Google+.
If you now want to learn more about a point of interest on Google Maps (and the location is lucky enough to have an icon) you can select it, click on the 'more info' link and be taken to it's usually empty page on Google+. So now on Google Maps instead of being able to click through to a useful Wikipedia article about a location you can instead visit an empty Google+ page.
To commemorate the death of the Wikipedia layer on Google Maps today will be Wikipedia day on Google Maps Mania. For the rest of the day I'll be posting links to useful alternatives for those of you who are lamenting the demise of Wikipedia on Google Maps.
For desktop users the best option is to visit the amazing Wikimapia.
Wikimapia is a collaborative Google Map, that allows everyone to share their knowledge about a location. Search for a location on the Wikimapia map and you can click on individual buildings, points of information and other features on the map. When you select a location on the map a sidebar opens with a description and photos of the location, as provided by the Wikimapia community.
On mobile the best option for those missing the Wikipedia layer is to visit Wikipedia's own Nearby.
Nearby is a brilliant way to discover what interesting places are around your current location and to learn all about them by visiting their Wikipedia page. Visit Nearby on your phone (or on your desktop) and you are shown a list of places around you with links to their Wikipedia (mobile) page.