I suspect that there could be all kinds of legal liability exposure if they offered retrofit parts for earlier bikes, unless they went to lot of documented effort to show that they did their due diligence in assuring that the new parts would not fail on older bikes... and that's really not worth it for the MoCo.
Imagine if they simply offered the new forks on the older bikes without extensive testing, and then someone's front end fails with the new forks, and they get killed? Lawsuit City! They are a HUGE target, and there are a lot of lawyers out there looking for someone with deep pockets to sue.
It's much easier for the MoCo to entice you to buy a new bike with all of the latest improvements, and I can't really blame them. They are a motorcycle manufacturer, after all... not an aftermarket parts supplier. I'm sure their stockholders would rather them sell lots of new bikes rather than lots of retrofit parts. That's just a fact of how any company's financial performance is measured by Mr. Market.
Ken