After observing a spike in customer support asking for help to un-install unwanted extension, the people at Google Chrome decided to enforce the policy which was announced in May 2014 to protect its Windows users by enforcing the developers to host their extension(s) on Chrome Web Store. With this policy, they noticed 75% drop in customer support help requests for un-installing the unwanted extensions. Soon, they will expand its protection to all Windows and Mac users in coming months.
For developers, they plan to continue to support local extension installation during development and also via Enterprise policy.
For more info, check out this post at chromium blog.