And both of those combine for "expense to operate, extra engineering headaches, maybe higher licensing fees with ASCAP/BMI/SESAC/GMR" and the corporate beancounters will say "turn it off".
HD has never caught on big time. The ratings prove it; in the vast majority of markets, HD channels are primarily used to feed analog translators.
The problem was in the beginning. Ibiquity was more concerned about intellectual property and milking licensing fees from radio stations, equipment manufacturers and anyone they could outside of listeners than they were actually getting it established. So, stations at least had a little fun experimenting with it, and could eat the initial setup costs. But there was a half-assed effort by Ibiquity to actually put it into devices that people could hear it on. Tabletop radios? Who buys those in the 21st century (though I picked up a used Insignia one at Goodwill for $5 recently). Microsoft put it in their Zune audio player, which was fine, but they had no idea how to compete with Apple on that failed product. Ibiquity should have been persuading car manufacturers to make it available, even in base models. But many of the car manufacturers were turned off by Ibiquity's shenanigans and licensing fees.
So again, a technology that was almost inaccessible to the public. At a time when satellite radio, digital music files, iPods, smartphones, improved streaming codecs, connected car entertainment systems and unlimited data plans were starting to become normal. And people ask why it wasn't successful?
Granted, the technology isn't perfect. AM HD is a dog. But the HD1's of FM stations do sound really good (when done right). Even the HD2 channels sound decent. It could have gone somewhere, and maybe it still could on a smaller level (but Xperi doesn't seem much better than Ibiquity). It could have worked, and it could have gotten interesting.
The FCC is certainly to blame too. They let Ibiquity try to turn it into a greed exercise, rather than let it develop and evolve like FM radio did (or chosen a less expensive, license-free technology like FMeXtra). The FCC, while sorting out their DTV plan, also could have done something like DAB using some of the TV spectrum. Obviously, a big chunk of UHF is now valuable real estate sold mostly to the phone carriers to help implement 5G, but what about low VHF? Channels 2-6 are just not very good for DTV broadcasting. Not many stations use it, unless out of desperation. It could have worked for digital radio, or even an extended FM band (or some form of both). But the FCC basically became lapdogs for corporate interests. They screwed the pooch on broadcast digital audio. And wound up with a dog.