I think that we may have more knowledge today than years ago about how you should practice and what you should do to not get injuried. So because of that maybe players can be able to play longer. I also think the game is a bit slower with the new ball and maybe that can make it is easier for players to play longer today, especially if you have an playing style where you block alot, play safe and have a good backhand. The play today is also more backhand oriented compared to before so proably players have a better backhand and do not need to play as much forehand as before which is harder for the body, so maybe players can keep up and play longer because of this.
When you had 5 serves and were able to hide the ball, and when there were more spin in the ball i can imagine it was easier to play longer because you could win more with the serve, spin and with less power. And the balls were shorter because of the hidden serve? Nowdays it is more power and less spin, and the ball is slower so players need to use the body more so maybe this will be a strain for the body so players can not keep playing as long? But because of the new ball i can imagine that players do alot more physical exercise which proably will prevent them from getting injuried, and can play longer because of that.
Maybe they could play longer before when there was speedglue because they got more help to get power in the ball then?
I also think that maybe how long you play also depends on how good new players there are coming after you. Waldner and Persson could play for Sweden a long time because there were not any new younger players that where better. While it is the opposite for the chinese.
I think that if the body is not injuried the europeans want to play even after the age of 30 since they have their peak then. Because europeans start to play later than the asians, it is not before after 30 years of age they have practiced enough to be at their best i think.
The asians start to play much earlier so they peak earlier in age. Maybe they are not as motivated after they have won alot already so they quiet? or maybe because they have played longer they have more strain on their body at an early age? or they do not think it as much fun because they have played so much?
I am speculating alot here so maybe im totally wrong! But interesting question!