Because in the first two expansion years the league had two divisions: An Original Six division and an Expansion division. The divisions did not play each other often. This created false parity, as it wasn't until the playoffs that it became more than obvious how weak the expansion division was (St. Louis, arguably the best expansion team during this era, had a 5-13-6 record against the O6 division in their inaugural season. Their record against their own division was 22-17-11. They were swept by the O6 division team in three consecutive Stanley Cup Finals).
After a couple years these rules were relaxed but the expansion continued, adding 2 more teams every 2 years until the 80s, when it absorbed the WHA. In the span of 12 years the league went from 6 teams to 21 -- an increase of 350%. With USA underdeveloped and half of Europe behind the Iron Curtain, where were all those new players going to come from? That's right: Lesser tier Canadian leagues.
It is against these lesser players (who normally would not have made the NHL if it did not expand so rapidly) that Esposito, Orr, Dionne, Lafleur, and ultimately Gretzky would absolutely feast on nightly. It took an entire generation for the global talentpool to catch up with the explosive growth of the league, and during this time there were a lot of very bad players playing very bad NHL hockey, as characterized in movies like Slap Shot.
