Chrome Frame allowed developers to scale the huddles of building different versions of app to suit the varied user agents, while ensuring better experience even for those still using old versions of the browser.
However, given the proliferation of modern browsers and more capable rendering engines, the challenge of users who were unable to move to a more capable browser is far lesser. And the usage of legacy browsers according to latest statistics is declining significantly.
What that means is that the leading edge browser experience have become mainstream, hence the need to retire Chrome Frame. Google will cease support and updates to the program in January 2014.
And developers who have hitherto employed the browser plug-in are required to instead prompt users to upgrade to a modern browser. While enterprise users who depend on old technology to run certain apps, are recommended to try Google Apps for Business, which allow employees to switch seamlessly between Chrome and another browser.