Probably because the iPhone 3Gs is MUCH faster and the voice command feature, which utterly spanks what the 3rd-party options offer, likely relies on that extra processing power. Apple could have probably put some sort of voice command on the other phones, but they would pale in comparison to what the 3Gs' can do, and Apple is all about the experience.
Come on man, you know this isn't true. Think about it logically. They won't offer it because it's a FEATURE of their NEW phone which they want to SELL. Take one or more features away from the new phone by having it be on the older phone, and people have less of a reason to update.
He11, voice command is a feature which has been around forever, and on the shabbiest of phones out there. Meaning, with processors that are years behind what the 3g has inside of it. You can say exactly the same thing for video, too. Heck, my silly Sony Ericsson W810i does video, minus the gimmicky "editing" feature which is touted on the 3G s.
And this is mostly why I won't be purchasing an iPhone this time around. I'll wait for next year, when the upgrade will actually be worth it. I'm thinking OLED screen, dual processors and running background apps... (hence the dual processor need) I mean, at the iPhone's current development rate, there's not many other places they can go with the current hardware/software. Something totally new is going to have to enter into the equation. And video, voice command and a silly 3mp camera don't count. These features on the 3g s are hardly any better than most other phones with said features on them, which have existed for years.
I'd personally stick with a jailbroken 3G until we see what happens next year. I find the current offering hardly worthy of spending $$ on .
Doug