 This is, in large part, to tone down the additional power potential of the "modern" rackets.However, it also is related to the tendency for different string materials to move out of place when subjected to heavy topspin strokes.Polyester is the string of choice today because of that resistance, despite its increased stiffness (harsher feel and more aggravating for the joints) and reduced tension-holding ability (versus a string like natural gut, which excels at that).