Supporting older browsers is an enormous drain on productivity, and it also influences design decisions for the worse sometimes by driving the design towards a lowest common denominator there the denominator is broken (e.g. terrible garbage collector in IE6/7)

It's time to move the web forward. I'd say this is an effort to turn Google Docs users into IE11, Firefox, or Chrome users.

It's not so obvious when you realize how much of CSS3 is unsupported by IE9 but supported by IE10. Or the fact that Google's policy for Google apps is to support the current major version and one back for all supported browsers. They've stated this policy for over 2 years. Now that IE11 is out, IE11 (current) and IE10 (legacy) are supported while IE9 (2 back) is dropped.

See also: Hangouts, which for some reason work well only in chrome, and only in chrome have an addon that integrates with your desktop.