Basically, there had never been any need to get that precise in the past, because every fuselage before it was so over-built. It always worked before, so why wouldn't it work now? But it turns out that when your fuselage skin is thinner and the loads it must withstand are simultaneously much greater, you actually do need to go the extra mile, because you're operating close to the limits of the material. The significance of extremely localized peak pressures in fuselage skins was not appreciated because nobody had to appreciate it before. It seems obvious in hindsight but experts were genuinely mystified at the time.