It's about reducing work for our developers who need to work around all the bugs in the older browsers. It's also about encouraging institutions to allow their users to upgrade, or at least support a dual browser strategy, thus improving the online experience for everyone. Using more modern browsers is free and not exactly a risky thing technically, even if an institution for some reason wants to stick to an old OS.
We shadow Google and they dropped support last year: http://googleappsupdates.blogspot.ca/2012/09/supporting-modern-browsers-internet.html
Note that dropping support doesn't mean Moodle won't work on those old browsers (it surely will!), it just means we won't bother too much if new bugs on those old browsers are discovered.