This policy is great for Chrome and Firefox, which automatically update to the latest version of the browser. This way, most users run the latest version of Chrome and Firefox, they get the latest features and security updates. The policy is not useful for Internet Explorer, which doesn't update automatically and has limited OS support. Starting from 2011, Microsoft updated Internet Explorer every year and each new release added support for more web technologies and made the browser faster. IE9 was released in 2011, IE10 was released in 2012 with Windows 8 and IE11 was released last month with Windows 8.1.
IE11 is only officially available for Windows 8.1. There's a release preview for Windows 7, but that's mostly for developers. Windows 7 and Windows XP are still the most popular Windows versions, so IE11's market share is pretty low.
According to StatCounter, IE10 is the most popular IE version (12.42%), followed by IE8 (9.45%) and IE9 (5.88%). They're low percentages, but this means that Google's apps only support one of the top 3 IE versions and less than half of the IE users will be able to use Google's services without issues.
"Google's test plans have been adjusted to now stop all testing and engineering work related to Internet Explorer 9 (IE9), as Internet Explorer 11 (IE11) was released on 17 October 2013. End users who access Gmail and other Google Apps services from an unsupported browser will be notified within the next few weeks through an in-product notification message or an interstitial pages with information about modern browsers and how to upgrade to them."
Supporting old browsers is a lot of work. Instead of spending time testing sites in old browsers and fixing bugs, you could add new features. Still, Google's policy seems to target IE's fragmentation and force users to switch to other browsers that are updated more frequently (Chrome and Firefox).
Android has a much bigger fragmentation problem than Internet Explorer. Supporting only the latest 2 Android releases (4.3 and 4.4) would mean targeting less than 3% of the Android devices. The most popular Android release is always 3 or 4 versions behind.