The iPhone 4S uses Apple's A5 processor. Which you know is more powerful than the A4 built into the iPhone 4—but it also has some weighty audio processing hardware baked into it. Specifically, the A5 is home to Audience's "EarSmart" technology, which uses a digital signal processor to remove background noise and secondary voices so phone calls sound better.
If you've never heard of Audience, don't sweat it: It's a start-up, but it powers Apple's noise-reduction circuity. Audience did, however, recently file its IPO. As part of that, the company revealed that its iPhone 4-era technology only works well when the phone is held near the speaker's mouth. The noise-reduction technology built into the iPhone 4S, however, is far better. It can cope with what Audience calls "far-field speech."
That is, in a nutshell, why Siri is officially only on the 4S. It's an app designed to be used from a distance: while the phone sits on your desk, as you're holding the handset to view a search result, or (perhaps less safely) while you're driving. The iPhone 4's audio processing abilities just aren't up to that task.
So, while hacks exist to install Siri on your jailbroken handset, or even legally, the plain, simple truth is that it won't work very well. Still, not to worry: it's not like Siri's that great anyway. [Securities and Exchange Commission via CNET]