As I said, I'm an armchair observer here, not a materials scientist. However, there are a lot of things that are absolutely designed and built to be single-use

As I said, I'm an armchair observer here, not a materials scientist. However, there are a lot of things that are absolutely designed and built to be single-use, especially in fields like space, where rocket engines have been single-use until very recently. As for weight, I think the reason that was important was because they didn't have a big enough ship that could operate a crane and haul the submersible out of the water. Really, the whole operation was a bad idea: this kind of exploration is expensive as hell, and the only way to cut costs (with current tech) is to do really dangerous stuff.