If I recall correctly, it has been reported that CF material was used to allow a more reasonable weight for a larger vessel-- one that could carry more people.
Another news story from the past (just prior to Pandemic):
https://www.geekwire.com/2020/oceangate-raises-18m-build-bigger-submersible-fleet-get-set-titanic-trips/
"This is shaping up as a rebuilding year for the nearly 11-year-old venture, based in Everett, Wash. The main task on the agenda is to build two new submersibles capable of diving as deep as 6,000 meters (3.7 miles), which is more than a mile deeper than the part of the North Atlantic ocean floor where the Titanic is resting.
OceanGate will take advantage of lessons learned during the construction of its carbon-hulled Titan submersible, which was originally built for Titanic journeys. Rush said tests that were conducted at the Deep Ocean Test Facility in Annapolis, Md., revealed that the Titan’s hull “showed signs of cyclic fatigue.” As a result, the hull’s depth rating was reduced to 3,000 meters.
“Not enough to get to the Titanic,” Rush said.
That meant the Titanic trips — which had been planned at first for 2018, then 2019, then 2020 — had to be put off until mid-2021. By that time, Rush expects the new submersibles to be ready to enter service. He said mission specialists who have paid more than $100,000 each to participate in the Titanic expeditions were “generally supportive but disappointed” by the delay.
Completing the latest funding round will open the way for proceeding with the Titanic trips and other expeditions on OceanGate’s agenda."
11 year old company at that point closed on $18m.