There are only ten vehicles in the world that can go 4000 meters or deeper and all of them are certified except Titan.
The submarine was made out of carbon fiber, which is fine for many applications, but not for subs. over use, the constant compression and de-compression can cause de-lamination. Where the carbon fibers and resin become separated.
Aircraft fuselage experience the same compression and de-compression, but the pressure differences are minor compared to what a sub experiences.
This was a highly experimental sub and should have been treated as such with no passengers.
The design was fundamentally flawed from an engineering standpoint.
They flatly refused to get any form of certification and the reason is simple. This sub could never achieve certification. They cut every corner they could.