If memory serves, the logic to drive the Touch Bar is something that came from the Watch, which also included the Secure Enclave and other features that were more mobile than desktop. If that was true, then you can't trust the USB cable alone (or Bluetooth) to maintain the trust that circuit needs, and a keyboard with a Touch Bar would require a new circuit or at least design change to only do the display part, not the rest. It makes sense they won't put it in standalone keyboards just yet.