It's also a question of how well it works. If there are plenty of times when Siri doesn't pick up what you are saying correctly, or at all, and/or if it takes a longer amount of time to understand and then reply, and/or something similar to that, then the user expeience wouldn't be a good one for many people, and therefore Apple might not want to make something like that available for those devices so that the user expeience doesn't suffer more than they are willing to accept. Just getting something working is basically only half of what plays a role in it all.
[doublepost=1466516859][/doublepost]
Well, if you are sure, then that does it, right?