"NBL crowds in Melbourne were fine throughout the 80s and 90s during the traditional winter season."
In the 1980s, there was no AFL, only state leagues (SANFL, WAFL, VFL etc.), with the expanded VFL arriving in 1987. The VFL (expanded or not) played games in Victoria predominantly on Saturday and Sunday afternoons (kicking off ~2pm), while the NBL played its games in Victoria predominantly on Friday and Saturday nights (tip off ~7pm). Late Sunday afternoon games, when they were played, usually started ~4pm or so.
In the 1990s, AFL arrived and played an average of 5 games a week in Victoria - the same as now. Those games predominantly were still played on Saturday and Sunday afternoons, still kicking off ~2pm. The AFL's experiments with evening and night games in the 1990s occurred at interstate venues (SA, WA, Qld and NSW). They played some Friday night games in Melbourne and these became more regular in the late 1990s. Thursday night and Saturday night games started turning up regularly in Melbourne from 1999.
The NBL switched to summer season in 1998/99, to get away from AFL, because the crowds had been falling through the 1990s. That partly reflected the expansion of the AFL and its increased capture of the live sport and tv sport market in the second half of the 1990s. In 1999 AFL really changed up the days and start times of games in Victoria as well as other states. As a result, this year we have a lot more direct clashes between game days and times for NBL and AFL in Victoria.
So you're comparing apples and oranges when comparing the AFL vs NBL situation of 1980s/90s with that of 2021.