Eh? In the 80s, I saw sold-out sleepers on almost every train I rode; sold-out coaches only once on the Coast Starlight.
I am told that the pre-Amtrak Bay Area to LA overnight train ran with something like 10 sleepers and 2 coaches. Meanwhile the Spirit of California was usually assigned 1 or occasionally 2 sleepers and ~4 Amfleets, assuming pictures on the web are representative.
Sleeper shortages pre-Superliner resulted in the Pioneer and the Mountaineer, among others, running overnight coach-only temporarily in the 70s.
Money is, of course, always an issue -- but it seems that sleeper shortages have been more common than coach shortages throughout Amtrak's history.
worldwide, it’s absolutely a new phenomenon. And their viability and recent interest in the USA (even when considering your point about the 80’s) is new.
People road sleeper trains 40 years ago, but that was still part of an overall decline in rail travel, that has since reversed.
So yes, sleeper train popularity all over the world (nightjet in Europe, and HSR sleepers in China) and reinvigorated interest in the American public’s imagination (perhaps due to interest in clean transport), is a new phenomenon.