As stated by Google "After February 2015, you will no longer able to view custom KML content in the classic version of Google Maps. KML is the file format Google Earth uses for the exchange of geographic information."

see https://support.google.com/maps/answer/41136?p=kml&hl=en&rd=1

Updates of corresponding pages will be provided in the near future, but until this would be done, you are advised to download the kmz file and use a local Google Earth installation to vizualise their content.

[Edit from the 10/04/2015] Openlayers maps are now used in replacement of Google Maps discontinued functionalities.