The iPhone 16 was the big "Built for Apple Intelligence" release, where they added hardware to support AI. So not too surprising to me that iPhone models before the 16/15 Pro wouldn't be able to support the new AI Siri.
The surprising part to me is that it took a full 2 years after the iPhone 16 launch to release the feature - and even then it's still a beta release.
I'm definitely looking forward to trying it out, but right now I'm still in the waitlist to actually enable the feature. I'm mostly just looking for a usable hands free driving experience compared the old Siri which I found to be almost completely useless. Anything beyond that would be a bonus. I have so little faith in Siri though at this point, though, that I am not expecting miracles.