The 9 champions looks bad, but I think only have 18 (17 if you counts the Sonics/Thunder as one team) different teams make the Finals over that period. Compared to 27/30 for MLB and 25/32 for the NFL. So over the last 35 years, 12 NBA teams haven't even made the Finals. Sure teams like the Raptors, Grizzlies and Pelicans haven't been around long, but neither have the Rockies, Marlins, Diamondbacks and Rays, but those 4 have all made the WS and the Dbacks and Marlins have won it.
The NBA has a really top heavy league. It seems like it has always been that way tough. I mean the Lakers and Celtics have a combined 33 of the 68 NBA titles and have a combined 52 Finals appearences.