At the risk of reopening that Pandora's box called "Clutch", I've often believed that ML pitchers are some of the most clutch people in the game. There are many pitchers with good stuff and good location but the difference between being clutch and not clutch is what separates the good ones from the bad ones.
Picture this: Every pitch a pitcher throws has the potential for disaster. He not only has to throw a strike, to be really effective he has to be able to throw it to the exact spot he wants to. Now put yourself in a tie or one-run game where one mistake, one error by inches, can be the difference in the game. The good ones, the ones who are clutch, can do that a lot more frequently than the run-of-the-mill pitchers. That's why they're more successful.