That digital AM thing they had going for while was great from my standpoint. No static, stereo, quality sound. WTAM and Radio Disney had it here. What happened to it?
By the time I had access to a HD Radio capable of receiving AM, these stations had already turned off their IBOC carrier. From what I heard, HD Radio on AM was limited to 1 subchannel, that being the main analog broadcast, and the quality was about the same as a MP3 at 56 kbps or a stream over dial-up. The only benefit was stereo and a slightly higher frequency response above 8 kHz (above 5 kHz if the station was already broadcasting an IBOC carrier).
There are many reasons why the vast majority of AM stations turned off their IBOC carrier. In order to shoehorn in the carrier, the station's sidebands had to be used (1090 kHz and 1110 kHz for WTAM). This resulted in increased interference to adjacent stations at night and even interference complaints from affected stations. In addition, about 5 kHz of the station's center frequency had to be cut, which resulted in the analog side sounding dull and muffled, and the IBOC carrier bleeding onto it with most radios, which resulted in a "waterfall" being heard in the background and dead center tuning in order to minimize it. (AM HD Radios had a 5 kHz lowpass, so the "waterfall" wasn't heard, but that reduced the audio on all AM stations to 5 kHz regardless if they had an IBOC carrier or not). Also, the IBOC carrier would only be decodable as long as the listener was able to receive the analog side practically interference free. Minor interference (like a pop) would sometimes be enough to disrupt decoding and cause the HD Radio to fall back to its analog until the IBOC carrier could be decoded again.
Transmitter maintenance and lower than expected HD Radio sales and adoption are likely the reason why most AM stations pulled the plug on HD Radio. Many stations never bothered to upgrade, due to the cost of the equipment and royalties that would probably take years to pay off, or was seen as not financially feasible. WTAM (iHeart) and WWMK (ABC Radio) were the only ones who could afford to upgrade their stations. WARF, I believe, was the 3rd AM station in the market to transmit an IBOC carrier, but it was short lived. WWMK had issues from the very start, and the carrier was off and on for about the first year, along with numinous issues on the analog side that were eventually worked out. All three stations stated backing away from HD Radio in 2013.
There has been talk about allowing AM stations to convert to a pure digital broadcast. Of course, that would mean that an AM capable HD Radio will be required in order to receive such a station, but without the analog broadcast, there would be an added benefit of better frequency response and better error correction to help overcome interference. How feasible this would be in today's streaming world is uncertain, but I highly doubt it would gain any traction.