Read on MacStories:

"Phone 4-era technology from Audience also has advanced noise-reduction, but it was only with the second-generation product (the one on the A5) that Audience perfected recognition/reduction at arm’s length — thus allowing for features like voice control through Siri. […] Integration of Audience’s EarSmart on the iPhone 4S’ A5 could explain why Apple decided to not offer Siri on older devices, and why it will likely only feature the voice-based assistant on newer devices going forward."

See, iPhone 4 users, Apple doesn’t hate you.