Hundreds of Thousands of Windows XP and Vista Users Won't Be Able To Use Steam Soon (vice.com)
Windows XP and Vista users have six months to upgrade their operating systems or get the hell off of Steam. From a report: "Steam will officially stop supporting the Windows XP and Windows Vista operating systems," Valve, the company that operates Steam, said in a post to its XP and Vista support community. "This means that after that date the Steam Client will no longer run on those versions of Windows. In order to continue running Steam and any games or other products purchased through Steam, users will need to update to a more recent version of Windows."
Because Steam is partly a DRM solution. Being able to run unpatched versions would allow for crackers to exploit unpatched vulnerabilities which could be used for piracy. Yes it's silly and ineffective like all DRM, but the big publishers, Steam's main customers, want to keep the illusion. Now I don't know what will happen to older games that don't run on Win7, but hopefully Steam will force the developers to upgrade them before the 2019 deadline.