The voice commands needed for SC are much different than the voice commands used in Elite. As well, the voice commands are tied to keystrokes and button assignments on the controller being used. SC uses different bindings for the controller than are assigned in Elite. It would be very difficult to adapt a Voice designed for Elite to be used in SC and since the voice provided for SC is free, why bother.
As well, SC is in flux and bindings/voice commands may change over time. Right now, the HCS Voices provided for SC are more of a "future market" free trial or alpha as it were, and if SC ever becomes a beta or a game, HCS would then be positioned to provide voice command support for it.