Scrappydog and others sell these pipes where the muffler and header pipe can be replaced separately. That would save you from replacing the entire exhaust every time.
-As for the cause, I think Irish is right about checking the spark plug color and jetting to see if your exhaust gasses are just too hot.
-Exhaust header cracks are usually caused because some of the other mounting bolts that support the weight of the pipe are loose or missing. Since you have replace the exhaust, you would have noticed if that was the culprit.
