Michael Coffey in the New York Times notes  that in the first 60 years of the 20th century, there were 4 perfect games in baseball, a game where a pitcher pitches a complete game and no one on the other team reaches first base. In the next 45 years, there have been 11, so the rate of perfection has roughly quadrupled. Why? He has a lovely incentive-based explanation:
Here’s a theory. In 1904, when Cy Young pitched his gem, the term “perfect game” didn’t exist. Young wasn’t aware until the last out that no one had reached first base. The New York Times ran a two-sentence story. Young was making $4,500 a year, about three times a skilled laborer’s salary.
By the time Bunning pitched his perfect game 40 years later, baseball was becoming a media business. Television had come on to the scene, prompting the Dodgers and Giants to move to the West Coast. Baseball underwent its first expansion, adding four teams to the original 16. After Bunning closed out his 6-0 win against the hapless Mets, he got $1,000 to appear on “The Ed Sullivan Show.”
TV money not only inspired franchise movement and expansion, it also energized players to form a union to get a piece of the action through free agency, which was introduced in 1975. That year, the average salary was $44,000; today it is over $2.5 million. This year, Randy Johnson will make $16 million.
What does money have to do with perfection? With free agency, players are aware that their best efforts will be fairly rewarded. They work harder — and they have the financial incentive to do so. They also know that extraordinary accomplishments are wildly celebrated. Cooperstown calls for a piece of the gear used in the game. If this does not inspire the kind of mental focus a player needs to go the distance, I don’t know what would. To make things easier, franchise expansion has brought many lesser talents to the major league level. Now with 30 teams, baseball offers 350 more roster spots than it did in 1960. Indeed, 5 of the 11 perfect games since expansion have come against expansion teams.
There is one other factor he does not mention. Groundskeeping and better gloves have improved fielding. I suspect a lot of perfect games were lost in the early years of the 20th century when hitters reached first due to errors or hits that might have been prevented by an Ozzie Smith.