My Dad used to fly the 737-200 and -300 for Delta, he says that the pressure differentail on the 737 is lower than thoseo f the 727 and 757 which he has flwon and the 707 evidently (which he never flew). Now I think the structural failures were caused because if the max pressure differential is lower than the other Boeing narrowbodies (which of course use teh same fuselage design), the guage of aluminum used is thinner, therefore will make is susceptible  to metal fatigue. That's my hypothesis. They haven't had teh problems on teh 737Ng's because the pressure differential is higher, so its gauge of aluminum is thicker.