Why is always on Hey Siri only available on the iPhone 6S?

At the latest unveiling by Apple and mentioned in this post, they announced a new feature on the iPhone 6S and 6S Plus: you can have 'Hey Siri' enabled always rather than just when plugged into a power source. After downloading iOS 9 to my iPhone 6, I optimistically checked to see whether this feature was present in the settings, but to no avail - 'Hey Siri' is still restricted to when the device is plugged into a power source. Yet, surely this is a software-related matter rather than anything to do with the hardware. Why is the ability to have 'Hey Siri' enabled when not plugged into a power source restricted to the iPhone 6S and 6S Plus? Has Apple made any explanation of this, or is there some other evidence which accounts for this?

It's just a matter of hardware. The M9 coprocessor in the iPhone 6s is efficient enough to keep the microphone listening all the time without murdering the battery, unlike the chips in older devices.

From the article your question links to:

With iPhone 6s, the device is constantly monitoring audio input in the background, a process made possible by the highly efficient M9 motion coprocessor embedded in Apple's new A9 system-on-chip.

The best you can do is to use an external battery pack and then it will work even on the six :)

In addition I actually like the way it works now on an iPhone 6, because when I go to bed I just say 'hey Siri good night' and I can verify that my phone is plugged in if Siri replies

Strange.. This morning I said "hey siri" with the intention that my Apple Watch responds, when suddenly, my unplugged iPhone 6 responded! Yes, shocked as you guys. I tried it a number of times and it is working. 'Hey Siri" works on an unplugged iPhone6! Don't know if I'm experiencing a "bug" or what, but I'll take it!