Well, in a nutshell, television is trying to cram 70 pounds of video into a five pound bag. Digital bandwidth for radio is a whole different beast. We're dealing with audio, which is a much smaller payload than HD video. Putting 2 CD-quality and one voice-quality audio streams into the allowable data pipe, when properly done, works out very well.Then you figure 90% of listening is in noisy cars by people who think multiplexed FM and compressed mp3s sound outstanding.
Of course, it depends on the source material. Ours is pretty darn good. We use a proprietary system that produces the same audio quality as we get off of the CD. Some stations save server space by compressing songs. Plus, we're country music. Yeah, it can sound great, but that's not as critical as, say, classical. Classical aficionados claim to have noticed when classical stations introduce a subchannel. I rather doubt anyone listening to us would notice. Our digital channel still sounds better than any FM, Satellite or mp3 you've ever heard. Most of the HD Radio stations in Detroit sound fantastic. But then, we're pickier as it's our stations the automakers sample when deciding whether or not to add the technology to automobiles, which is the make-or-break for HD Radio.
I can't speak to television's monetization of their subchannels, but they do seem to run plenty of spots. Our second channel is commercial free. Our third is ad supported, as is WOMC-HD3. Both turn a profit, though that's not terribly hard as operating expenses are rather low.