In May 2014 we implemented a new policy to protect Windows users by enforcing that extensions be hosted on the Chrome Web Store. The results were encouraging: we saw a 75% drop in customer support help requests for uninstalling unwanted extensions. We will expand the reach of this protection to all Windows and Mac users in the coming months. Starting today we will begin enforcing this policy on all Windows channels. Mac will soon follow, with enforcement for all channels beginning in July 2015. For developers, well continue to support local extension installs during development as well as providing integrated install flow from your own website.”]
Source: https://blog.chromium.org/2015/05/continuing-to-protect-chrome-users-from.html