Apple: We Have No Plans To Bring Siri To Older Devices
The answer may lie with another hardware component unique to the iPhone 4S. iFixit re-examined the components of the 4S they took apart. They noticed an infrared (IR) LED that is almost always emitting IR to see where the user is relative to the 4S. iFixit speculates this is to provide information for the Siri software to determine when you are about to ask it a question. You can watch the video embedded below for a detailed explanation.
Video source: iFixitYourself