I'm sure iPad 2 doesn't have the capabilities to run Siri because they didn't know what it would take to run it 2 years ago. So, they couldn't include what was needed. They obviously recently discovered how to make it work, and needed to include hardware as well as a major upgrade to the iOS.
With the great leaps in technology, I'd be surprised if iOS 6 will even run on the old iPad 2, let alone Siri.
I would be surprised if iOS 6 did NOT run on iPad 2. Its still sold.. still supported.