Although one might think this is a reaction to the Internet Explorer's vulnerabilities notably exploited in the recent series of Chinese-based attacks against Google and 30 other tech companies, which Microsoft has since patched, the truth is Google has already done this with many of its other products. Google's Orkut and YouTube started phasing out IE6 support about six months ago and Google has been using Gmail to convert IE6 users to Chrome for over a year.
Google's stance on IE6 varies from Microsoft's because the search giant does not need to support Windows XP, the operating system with which IE6 first shipped, as long as Microsoft (which will support XP and IE6 until April 8, 2014). Google can thus pull IE6 support on its many Web properties and urge users to upgrade. Microsoft, on the other hand, which has stated time and time again that it wants to see IE6 disappear as much as anyone else, won't force anyone to upgrade (though it's worth noting that the software giant's Office Web Apps won't support IE6 either, just like Google Docs). Instead, it says the decision is ultimately up to the user, touting IE8's many features over IE6, particularly in the area of security, in an attempt to push users to upgrade. IE6 ended 2009 with a market share of 20.99 percent.
Source: ars technica