I haven't verified this at all, but I have a theory. Here's three key pieces of info to know beforehand.
- The CEO was not a billionaire. I've seen many people claim he was, but he _seems_ to have had a net worth of around 12 million. Certainly rich, but hardly anything close to a true billionaire
- Steel is the preferred material for these types of subs, because it's very good at handling the constant compression and expansion that would be happening at the molecular level. However, steel is heavy.
- Carbon fiber is light
Had they built this sub with the correct materials, it would have been heavier. It being heavier would have increased costs: they'd need a larger support boat to carry it out to sea, more fuel, possibly more staff on the boat, stronger/heavier support equipment, etc. So, in short, my theory (and I want to stress, I haven't seen this verified anywhere) is that they went with carbon fiber because it would be cheaper in the long run.