First of all, the Pistons have been down big in the playoffs before. Last year against Philly, the Pistons got down big in one of their games. And, in their championship year in 2004, the Pistons got down by double digits to the New Jersey Nets in game 6, an elimination game.
However, it's not always "not playing the full 48 minutes" that gets the Pistons in trouble. Basketball is a game, a sport. Sometime a good team can get a lot of good open looks but the ball just doesn't go in the basket. And, as in the case of Milwaukee the other night, sometimes the inferior team just shoots the heck out of the ball. I mean, Michael Redd was hitting fade-away 3-point shots with a hand in his face. He wasn't even looking at the basket and his shots were going in all net. It happens.
Now, it's not to say that the Pistons couldn't have played better defense, especially in transition in the first halves of the Milwaukee and Phoenix games. They most certainly could have. But, you gotta give credit to those two teams as well for playing extremely well and shooting the ball extremely well from the perimeter.