80s, 90s and even the early 00s still had heavyweights, the lack of heavyweight boxers is why the drop off happened. The lack of star power today has nothing to do with the drop off boxing experienced prior to this point, as in viewership was already declining. It's all about the heavys.
It's also why boxing seems to be generating interest now with Wilder, Fury, Parker and Joshua.
Wilder-Washington Peaked at 1.86 Million Viewers; Averaged 1.76M - Boxing News
The PPV Numbers For Joshua-Klitschko Are Bloody Staggering | Balls.ie
People want to see the freakishly large dudes throw hands.