There now have been five perfect games thrown in the past 4 seasons (2009-2012) and six if you count Armando Gallaraga's game in 2010. (I am not including him to start an argument, just for point of reference.) That means that 23% of all perfect games have been thrown during the past four seasons (26% if you count Gallaraga's).
I cannot help but think that strike zones widen considerably as a potential perfect game progresses, but it does not seem to play out in the form of huge strikeout increases (Halladay's 11 Ks is not hugely out of the ordinary and Buerhle's 6 Ks and Braden's 6 Ks are nothing big.) The other two have big discepancies (Cain's 14 Ks and Humber's 9 Ks are much higher than their averages). But perhaps an increasing strike zone and more aggressive hitting make it easier to take the complete game the distance. Maybe part of the answer is that batters do not want to be seen as the guy who took a close ball 4 to break up a perfect game. (I wondered this specifically in the 7th inning of last night's game when Altuve struck out looking and Lowrie struck out swinging on back-to-back 3-2 counts.)
Regardless, unless it is a fluke, the achievement risks falling into the same hole that swallowed up the mystique of the 60 homer season.