The reason, is that without the full communication between camera and flash, the flash doesn't know what the shutter speed it. So it wouldn't know when the 2nd curtain will be closing...so it can't sync with it. The basic radio triggers only tell the flash when to fire...nothing else.
Yes Mike. Thats understandable. But 2nd curtain sync doesnt need any special coded info to be tranferred via radio. The cam body manufacturer can just ensure that when an off cam flash is attached the 2nd curtain pop signal is sent at the last moment just before the shutter closes. Its just a burst signal. But when that signal is popped can be decided by the cam. Im sure its just some programming into the EPROM onboard the cam, why would they hold back such simple things.