The main problem with steel is that it can only withstand such pressures if it's of spherical shape (i.e. a bathysphere). This usually only leaves enough room f

The main problem with steel is that it can only withstand such pressures if it's of spherical shape (i.e. a bathysphere). This usually only leaves enough room for two to three people (e.g. https://en.wikipedia.org/wiki/DSV_Alvin https://en.wikipedia.org/wiki/DSV_Alvin). So it seems it's more a matter of cost and passenger space than just weight alone.