[waxx]

i think there are several reasons:

better arrangment (not so dense like now is normal) it's like because you can record 96ch or so in your daw you've got to use them all these day's. Most of my own stuff goes about 12-24ch...
good gain staging (some 'engineers' still don't get it)
more instrument or even a whole band capting with one mic on the right spot in the right room and more room/less close micing
much less compression and limiting, especially on the masterbus and during mastering
better musicians (or you heared they sucked, no autotune, beat detective, midi and fast and easy editing like now).
better sound engineers (the machinery was very expensive and often more difficult to operate, so even for demo recording you had to have skills)

tape did also indeed hide a lot of details, so also the little mistakes. Digital is often too revealing. It helps to use a LPF with a 12dB/octave slope arround 14K till 16K on the masterbus to get the tape rolloff effect that hides a lot without loosing to much clarity (i do it often)

and don't forget, we only remember the good things of some era's of the past, I'm rather sure that in the 70's there was also a lot of shit-music arround (i wasn't until the end), you only don't remember it anymoree because it's not worth it. I also know a lot of lofi production of that time (i talk about a lot of reggae and afrobeat for instance) that's actually good music, but lo fi due to the lo fi budgets ... But the mainstream listeners may not know that. Not everything produced in the 70's was sounding better.