Film director James Cameron has expertise in designing and testing these submersibles, and he has many criticisms of the design of the sub that imploded, and of the hubris of the CEO who ignored repeated safety warnings from the diving community. He also mentions that the sub seems to have been attempting to resurface when it imploded, suggesting that they were aware the hull was starting to fail.
Cameron did it in a sub that was tested and certified. The Titan sub was not actually tested or certified, because that would have been expensive.
Hell, Titan’s view port was only rated for 1300 meters, not the 4000 meters of the Titanic wreck.