Because clay is a quirky surface. We hear about clay court specialists, but not hard/grass court specialists.
Are you serious ?
Never heard of grass courts specialists ? You just grasping for straws at this point. A whole cast of players (let's call them "serve & volley players") feasted on that surface because it fitted their game (same as heavy lifters feasting on clay because it fits). Sampras WAS a grass specialist (same as Roddikk, Henman, Edberg, Becker or Ivanisevic for example) because of his top notch serve and volley game. Anybody who saw the '99 Wimby final against Agassi would told you that this type of game wouldn't have been possible in other surfaces than grass because that was the epitome of grass tennis.
LOL at you dudes "well clay is so special, we shouldn't count the victories on that surface". Clay is NOT a special surface, it's just another surface with its characteristics (high rebound, slow, lift-prone), the same as grass with its characteristics (low rebound, ball having the tendency to burst forth) of same as indoor, same as hard-court, same as carpet... And on every surfaces, you have a different way to move (slightly or not), you skid easier on clay, but you can also on hardcourt. Grass is slippery (remember Wimby '13) so you got to watch your steps and be low to hit the ball. Grass tennis (and indoor / carpet) is (theoretically) fast with short rallies and flatter strokes. Clay tennis is (theoretically) slower with long rallies and you need to give more spin to your balls for it to be more efficient as the rebound "slows" the ball. Tactics are different too as you need different tools to overcome your opponent because the characteristics are different. Hard-court is in the middle somewhere. But no clay is too special so we need to scratch that surface.
It's not clay that is too much present, it's grass that is not enough because of the amount of hard-courts everywhere. And also because of the higher cost for grass and clay courts maintenance.