The change from 15 to 21 put more emphasis on physicality. I think until then players used counter-attack a lot more and reading of your opponents moves was much more prevalent, much like a chess game.If you want a good example look at Icuk Sugiarto v Morten Frost. Playing at a higher pace could have terrible consequences in the old 15 format. Of course at top level the players have that ability, but I think it's far easier to climb the ranks on physicality now than it was 30 years or so ago.
