It seems that it can be put on iPhone 4 as well (http://venturebeat.com/2011/10/15/siri-iphone-apple/)
Why shouldn't all software anyone writes be given away free to anyone who has hardware that can run it?
However, from my experience so far, I suspect there are a number of technical reasons:
1. The microphone quality and position of the user relative to the mic.
2. Service Scaling - it is only just keeping up with the current user load.
3. Optimization - it's only just good enough as it is. I'm pretty sure Apple will be working hard to make it more responsive and more capable. More platforms would divide their resources. If SIRI was mature, this wouldn't be such an issue, but it most certainly is for now.
4. Integration - this is related to point 3. Siri is integrated into the phone well enough to be useful. It's good for reminders, notes, and meetings, plus the odd calculation and find my friends request. It works well enough to make a genuine difference to these kinds of task, but it has a long way to go before it is a universal UI.
Since more people have phones with them all the time, it makes sense that much more of this kind of assistant functionality will be accessible on the phone platform. Therefore, Apple will be focussing on maximizing it's utility for phone use cases first. See my earlier comment on divided resources.