Apple rarely backports new tech to older devices. It probably could run on the older chips, but it doesn't look like Apple isn't going for the widest possible rollout. They are launching it for the newest chips only, and are not wasting time porting it to old chips, when they don't even know yet how it's going to scale. So right now they are focussing on English speaking markets and newest devices only.
They are skating to where the puck is going to be, and in a few years no-one will care if their tech runs on the A16 or not. Right now they are focussing on getting this thing launched, and backward compatibility would only slow them down.
And its a selling point to get people to buy new iPhones, so it's win/win for Apple.