Any reason Google's Siri counterpart would be limited to new versions of the OS? Most (all?) of Google's core apps are available on older versions of the OS. I

Any reason Google's Siri counterpart would be limited to new versions of the OS? Most (all?) of Google's core apps are available on older versions of the OS. In fact I'm not sure I understand why Siri isn't available on the iPhone 4/iPad 2, etc.