Here is a steel pressure tank imploding. It was under only 14.5ish psi. The Titan would have experienced HUNDREDS of psi. Also, it's pressure hull was made of Carbon Fiber, which is notorious for failing catastrophically with zero warning, whether due to a manufacturing defect like a simple nick, or just going beyond what it could physically withstand.

The vessel was NEVER tested to 4000 m. It was probable that it was always going to fail at the depth needed for the titanic. There was no warning for the crew and passengers. One instant they were descending just fine, the next they are human bisque.