First, he was an aerospace guy and several things he's said make me think he was sort of chauvinistic about deep sea exploration in general, stuff like "It’s perfectly fine. Having all these certifications for airplanes is one thing, but the carbon fiber was perfectly sound."

Second, his business model, taking four people down with him in something other than Cameronesque claustrophia, and doing so without the cost of owning a proper launch vessel, instead renting any ship that could hold and then monitor his launch sled, meant it was critical he make something big and light, by deep sea submersible standards, that was at least nominally expected to handle the load. Shit, I guess in some sense, he did, since it went down and back two or three times or whatever. At the absolute best, though, he'd invented a disposable sub, and he clearly didn't worry about that limitation any more than the rest.