It's not expansion, there are more teams, but there are more players now as well. Players don't get taught the right way at any level, and leave college early. They are brought in based on potential, not polished skills, and the coaches aren't all that proficient at teaching the basics, and most players don't realize how deficient they are.
Basketball was played at a much higher level in the early days, but with inferior athletes. Then as today, the key to winning is a big man in the post, except when there are no dominant big men on the other good teams. Boston won all those Championships because they had Bill Russell all those years. But there aren't too many 6'11" athletes who had the same preparation growing up that Russell did. He spent 4 years in College before he came out. Bill Russel would have been drafted straight out of high school if he was coming out today. No one gets the training or the understanding and there are too few big men around for it to become proper knowledge thoughout the sport.
The athletes today are better in every way, except that the are dumber, more arrogant, less disciplined and the league has become more lax with it's rules and enforcement of double-dribbles, traveling, etc.
They make it easier to score these days, no more hand-checking, the three-point shot.. but teams score less because they don't play team ball anymore and players don't shoot from outside as well as they used to because they all want to be able to drive to the basket. Short guys used to stay out of the paint, because they would get clobbered, so they learned to shoot from outside. Now, they have learned to drive the lane, and the league softened the rules so they could do so without worrying about needing stitches.
Crap, they don't even know how to defend anymore which is why they now allow zone defenses! The game is watered down in terms of quality. It may be a bit more entertaining, and flashy, but fundamentally, they suck.