This wasn’t the sub’s first trip to the Titanic, so the previous successful dives probably instilled a false sense of confidence in the design. The problem with carbon fiber in this application is that it degrades with use, and the degradation doesn’t occur in a predictable way. It might work a handful of times, and then spontaneously fail with little to no warning. Additionally, while strong in tension, carbon fiber is weak in compression, and in a situation where the vessel has higher pressure on the outside than on the inside, that puts the hull in compression. In aerospace applications where the pressure is greater on the inside, carbon fiber makes more sense. Steel and titanium are much more predictable for deep sea applications due to their fatigue characteristics and compression strength.