Probably for power reasons, much like the most recent iPhones can be controlled by ; Hey Siri". However older iPhones can always listen too but are required to

Probably for power reasons, much like the most recent iPhones can be controlled by ; Hey Siri". However older iPhones can always listen too but are required to be plugged in to do it because they're using their main processor and doing it at a software level. In short, always listening isn't difficult on non-battery devices, it's just a software problem.