In the 70's...you had 18 teams prior to the ABA folding. That took the league to 22, but also created an influx of talent. The ABA had a lot of talented forwards and guards...maybe more so than the NBA at that point. As a result, you had a lot of very closely matched teams...a lack of meaningful FA movement prevented teams from building a Super Team.
The talent was distributed more evenly and there really wasn't a mechanism in place to allow teams to build other than through the draft and trades. Look at how long it took to build the Bad Boys. The foundation was built in 81-83, but they didn't have all the pieces to actually contend until 86-87 and then it took a couple more years of tinkering to get over the top.