Because they can pop and the decomposition may not be terribly good for the snake. (decomposition is basically the breakdown of cell membranes and the digestion of it by bacteria... which breed quickly and cause illness... which is why we don't eat spoiled food either.)

I had an almost-pop once... but realized something was off when I picked it up and the dead rat POOPED and some guts came out with it. I defrosted another one and threw that one away...