the differences between nba now and then

[superheadcat]

i remembered watching on espn classics about 2 months ago a game between piston and nugget in 1985(?) season. it went 5ot and piston won 183-181. in nba today, a game of 83-81 is normal (look at the latest champ).

which prompts the question: what makes the difference?

i am not an nba fan long enough, nor knowledgeable enough to give more expert insights, but all the possible reasons I can think of are:
1) players' skill and ability
2) teams' playing style
3) the rules the games are allowed to play by

but, for the player skills, it is hard for me to imagine that the players 20 yrs apart will have such a huge gap in the skills of scoring points.
for the playing style, are the teams 20 yrs ago playing no defense? i am not sure, but isiah thomas did play in the game mentioned above.
for the rule change, if that is the reason the scoring drops so significantly, why did the rule change happen in the first place? nobody wants to see players scoring?

any opinions?