Not to go all old school, but recall that Siri was developed for older apple iphones, and was going to be fore black berry and droid phones as well.
I get that what you get is what you pay fo,r but in the world of software thats not always true, its not impossible to add feature/functionality to a device that didnt ship with it. Noise-cencelling mics or special 'x' model A5 chips?. The point is the software worked on previsous generation devices, the reason they dont now isn't really technical, its marketing, which is just a choice Apple had to make. make it what they wanted, ovver a possibly lesser robust feature on earlier platforms or not at all. Looks like they chose not at all. I wouldn't be my choice but it all comes down to marketing.