The metal pressure vessels are very negatively buoyant. The bathyscaphe Trieste used 32000 gallons of gasoline to achieve positive buoyancy. Modern DSVs use incredibly expensive and fragile syntactic foam.

A CFRP pressure vessel can be naturally positively buoyant, or much closer to it. Titan's dry weight was less than DSV Limiting Factor, despite having much greater internal volume and carrying 3 more people.

Also, being able to load and launch in the water, rather than loading on the deck and needing a human rated crane to launch as most DSVs do, probably saved a lot of money operating Titan.