I just read a recent issue of Craig Wright's Diamond Appraised newsletter (it's a subscription service, but it's well worth the price) and he proposed a theory for the recent increase (in baseball time) in perfect games that makes a lot of sense to me. In short, it's due to the fact that there are fewer perfect games. I'll let Craig explain:
The further you go back in history the more the starting pitchers were trained to either finish the game or go as deep as they could. When you talk to the old-time pitchers it is quite clear that they enhanced their durability by coasting when the game situation allowed them to. You might go out there with great stuff but with the mentality of wanting to be sure to maximize the length of your outing, you might coast on a few batters in the early innings and get nicked for a hit that didn’t so much hurt your chances to win but which meant losing a no-hitter or a perfect game in the early innings before you realized the magical opportunity was there to have a special game.
Today, pitchers tend to give you their best stuff pretty much on every hitter and let the bullpen pick them up when they get tired. Under that philosophy, you are more likely to realize that you have your A-game going at a point in the game where you have already laid the early groundwork for a no-hitter or perfecto. The fact that you aren’t generally trained to complete a lot of games is not likely to hold you back in those special games. If you are pitching that well, you aren’t going to have to throw a lot of pitches to complete the game.
I think it's a nice theory that makes a lot of sense in the context of baseball history. To put a point on it, Dallas Braden's perfect game was also the first complete game of his career. Same for Armando Galarraga.
Read more great baseball stuff at The Hardball Times.