at 3800 M bellow the surface, the pressure is 5900 PSi (Pounds per square inch). In order for a structure to survive it, it must be spheroidal or a cylinder with spheroidal ends. That has been the bases for submarine design forever.
Deep sea, means that cylindrical with spheroidal ends. and in thick metal. As a result, as they grow in size, they grow in weight. That makes maneuvering, dropping to the bottom and floating at the surface of the water very difficult. Transporting the vessel to its intended dive spot, is hard. Hoisting it via cranes is hard and so on.
So, existing models are one or two persons. And they require the one or two crew men to lie down.
The designers of this vessel opted for Carbon fibre.. It is a weave in 1 K, 2 K, 3 K, 4 K, etc. and a resin that is poured around the weave and then cooked into dryness. The end result is a structure very strong, yet fairly light... most modern cars are now a heavy dose of carbon fibre.. as is a number of other products.
connecting to carbon fibre is difficult. Applying a tensile force is risky (pull force). Applying a moment is risky (bending).. It does not fatigue well, it is susceptible to bubbles and voids and so on. In many instances, metal has to be bonded to it, to further add pieces. In some ways carbon fibre/composites are magical, in other ways, they are a risk. Ocean water is brackish, salt.. that means Chloride ions (Cl- ion).. that can cause corrosion and pits. And dissimilar metals/materials will form a battery. Low cycle fatigue and fatigue loading is difficult to predict.
The choice of carbon fiber was to make a bigger vessel, that was still light. Allow for multiple passengers who are not lying on top of each other.
So many mistakes..
@Stylizer1 is correct. In time, it will be okay We will have accidents to learn from. Where I believe that Stylizer is wrong? it is un-necessary to learn from mistakes for indulgences and not true benefit.