Anyone know why they built it out of carbon and titanium in the first place?
Deep submersibles are extremely weight critical, meaning the weight to displacement ratio. Just like a diver, they need to be neutrally buoyant. The greater the design depth, the thicker the hull wall has to be for a given material.
The weight and displacement of every screw and length of electrical cable was tracked during the design process, even in the 1970s. That was really labor intensive before solids modeling and finite element analysis software became affordable and usable without months of training.
Very expensive and bulky syntactic foam must be added once the components (hull, batteries, systems, etc) and crew weigh more than the water displaced. That increases the power (batteries) and propulsion requirements creating a cascading effect on the cost and size of the boat.
Another problem is dry weigh. The heavier and larger the boat, the more complex and expensive the handling system becomes. At some point the machinery to get the boat in and out of the water in reasonable open sea conditions makes it difficult to impossible to use a vessel of opportunity — a vessel that can be leased around the world. A dedicated ship and professional crew is an enormous expense unless you can keep it busy (making money) most of the time.
Leased or owned vessels aside, the costs continue regardless if you have to wait for better weather/sea conditions in order to get the boat in and out of the water. This is a HUGE cost justification to design the handling and sea keeping capacity of the vessel to handle higher seas. This issue alone has nearly replaced manned submersibles with ROVs in science, military, and industrial applications.
All this is why the great majority, if not all, the hulls for greater depths are spherical. Spheres are the most efficient geometry for equal distribution of hydrostatic forces. The wall thickness has to increase around hull penetrations for electrical connectors, hydraulics, viewports, and hatches. This makes it critical to do the best engineering and machining job possible to make the strength around all these penetrations as equal as possible to the rest of the hull.
Very small deviations from the "perfect" sphere makes huge differences in the engineering calculation — meaning radius, wall thickness, and material homogeneity.