The problem is that there is no way to monetize to the point of profitability HD on FM, there is not enough market penetration of the radio (although Toyota is making it standard equipment) and there is certainly no way to do it on AM at the moment.
For the cost of the equipment and upkeep the powers that be don't want to be involved in AM HD, and I used to love WBZ-A in HD
The powers that be seem to think the investment in HD on FM is worth it, and with a 2,3,4 and sometimes 5 sub channel available for little to no outlay once they have bought into HD-1, they can just put the AM audio on a FM sub and save a ton of cash.
The radio in my daily driver in NH has no AM stations preset, I have WRKO and WBZ-A preset as a HD-2
AM Stereo never caught on ( I had a receiver in my 1992 Cougar), the expanded band is a joke, Translators are just life support for most stations, and with the land under towers being worth multiples of the value of most AM's, even a conversion of the whole AM band to digital would still not solve the problem of retaining or placing antenna farms.
And none of the above deals with streaming audio being the emerging preference of people under 34.
I don't even have a SXM receiver in my old man middle age crisis car here in Florida (2007 Mustang GT Ragtop w/ 11K miles on it) I am already paying for 3 subscriptions that give me streaming... one is tied to my amazon account, another on my phone, and the third on my wife's phone. I ripped out the factory radio (the CD changer died, a known issue) and put in a new double DIN head unit with Android Auto , Apple Carplay, bluetooth and hands free. When I plug my phone in I can stream SXM from the radio screen , changing channels, etc, or I can use Amazon Music, or any of the radio streaming platforms. Even on my motorcycles I can bluetooth or AUX input my phone and one of my GPS units has a couple of thousand MP3 music files on it.
