Musk explains why Steel is the best material to work with for any engineering project, where strength, lightness, durability, reliability and reusability is required.

Carbon fibre was discarded early on, aluminium too.

In Musk's own words
>'We went from 120 ply to a single ply, $130 a kg to $4 a kg and had a vehicle that had the same strength, was lighter, can be reused and has practically zero chance of catastrophic failure'

https://x.com/elonmusk/status/1798937746624631249

The titan sub disaster forced Space X too look again at Carbon fibre, and they soon discovered that the risks far outweigh any benefits, and many of the claimed benefits turned out to be simply untrue. Steel beat Carbon Fibre hands down on every metric they looked at.

>''Carbon Fibre is great for single use applications, but even then it comes with massive fundamental risks, any tiny flaw in the weave, an air bubble for example, can cascade into catastrophic failure, at the end of the day it's just very expensive reinforced plastic.''