The Pancacked (shortened.. i.e. no skirt) Piston
I'm commenting not on why 4 strokes are better (I have both 2 and 4 strokes), but I'm commenting on why the 4 stokes became much faster with the '97 Yamaha yzf.
When Yamaha eliminated the piston skirt on their yzf 400, they gave the engine the ability to accelerate MUCH more quickly and also the ability of the engine to obtain a much higher RPM. Imagine if engineers could do the same with a 2 stroke piston. The difficulty would be in timing the gas/oil mix but I hope engineers are working on that.