I have to go with the random odds. Most WSs are pretty close to the same and there just happens to be a who lot of 738s making a lot of flights, thus getting inspected a lot. When a crack appears in flight, they put down and R&R the WS. There are so many 738s flying around that finding a replacement WS on short notice is usually not a problem. As I recall, the R&R procedure takes about 60-90 minutes and is a non-event for for the tech folks. That said, I'm not a techie and maybe I'm blowing smoke. All of that said, random cracks in a layer are FAR different from a full thickness fracture or complete blow out. Those events are exceedingly rare.
