HD (High definition) is about getting great quality to that of regular cds to reveal more of the music making and venue but in an age that is more focused upon convenience, in the 2000's the iPod with its hundreds of poor quality lossy 128kb files or today with streaming, eating your data allowance on your smartphone when you have no right to expect anything to be there, it didn't become a everyday thing beyond musicians and hifi fans.
Part was and today remains it takes over £800 to get a super audio cd player and then only a few handfuls of discs are issued per year which was why I spent money improving on the regular cd reproduction even though I have a Super audio cd player in the hifi.
HD Downloads were easier in that you selected them from the store and most digital audio players of the portable sort would just play them - the players could be had for around £100 upwards but much of that marked has moved from playing downloads to streaming music.
It's a pity as while as with anything based upon digital files you need to keep them backed up, they leave your mobiles data allowance untouched and as you own them, there's no question of a performance becoming unavailable in the way sometimes labels and artists pull things on the likes of Spotify.
As the number of super audio discs Chandos puts out has dropped in the last two years not helped by there being only a handful of plants world wide that can make them, I can see a return to more HD downloads not least where I may have "classic" performances on cd or even record