This decision by Valve to do with the limitations when working with Google Chrome.
Valve announced that it intends to discontinue support for the operating systems Windows XP and Windows Vista. It will happen January 1, 2019. This means that on the specified day the customer of this service will not run on vysheupomjanutye OS versions from Microsoft. Until January 1, Steam will continue to work but will no longer receive updates and new features.
“To continue to use Steam and run with it, games or other digital products purchased in Steam, users need to upgrade to a more modern version of Windows”, — stated in the message support service.
The solution Valve is connected with the limitations when using Google Chrome, which also ceased to support old Windows. In addition, Valve needs some security, just appeared in Windows 7 and later versions.