The 6S handles the "hey siri" function with the low-power M9 co-processor, rather than the primary A9 processor. This means having it available without the power connection won't have a huge impact on battery life. Earlier phones can't do it that way, so if Apple turned the feature on for older phones, they'd get an influx of complaints that iOS 9 is a battery killer on those phones. They'd probably get a lot more complaints for the battery problem than they will for the hey siri feature only being available on the 6S. Presumably if you jailbreak an older phone to get the feature, you'll also get the battery drain to go with it.