The style of play and player quality may be a factor too. I've watched games from the 70's or 80's (even those with all blacks), and teams played real team basketball. All 5 players touched the ball, isolation plays were quite rare, even superstars had discipline and worked hard (no favoritism). NBA games were just enjoyable (even in the 90's), and most players were good role models (other than the Bad Boys). It got really ugly when HS players started dominating the hardwood. These players (and the 1-and-dones) tainted the style of play and lookout of the league. They brought to the league rap, streetball, and tattoo flavor. Attitude problems of certain players have plagued the NBA's images (like Iverson), and in recent years, several have endured financial problems and issues with the law, great role models they are indeed. Lastly, Referees have gotten so crooked over the years, they often decide which teams should win.
I agree with WA33 the decline white American players has weakened the NBA's popularity overall (which was also evident in the late 70's). Lack of White Americans have lessened the leagues appeal to many white folks. I always find it odd when a white kid idolizes a black sportsman, when he can NEVER be anything like him. Even if most won't admit it, many prefer to identity with someone of their kind. The NBA is still enjoying decent rating thanks to the Black population, DWF's, and overseas (like China & Philippines).
Despite it's claims of being Global, the league still maintains it's agenda (since the 70's) of being black, letting in some whites (US & foreign), and even lesser OTHERS.