Contracts. These providers will not let VOOM carry only the HD channels. They also want V*, E*, D* and cable as well to carry their regular SD channels. It will be interested to see what's going to happen as HD channels are in demand and growing and the SD channels eventually are phase out.
According to your channel lineup from the source code channel 281 is supposed to be INHD. I wonder why VOOM selected this channel to use for boxing ? Maybe this means INHD will be added after the boxing ends on Saturday.