Even if extra channel space is available - it is usually not enough for the very wide bandwidths of HDTV. As an example, the current NHK satellite broadcast system in Japan (the only "in-service" HDTV system) requires 20 MHz, but only has 8.15 MHz available per channel from direct satellite broadcast.
Thus, some type of compression is typically required. Interestingly enough, although these compression schemes result in analog signals - they are digitally implemented. Thus, the line between "digital" and "analog" HDTV begins to blur.