Why: Making any sort of thick, strong plate metal is seriously non-trivial, both in terms of the manufacturing facility required, and the metallurgical expertise to actually do it. The CSA had very little heavy industry compared to the Union. Boiler plate (widely used, in a steam-powered world) would be about the only armor-ish product that the CSA could manufacture for itself. (Similar for hull plates for a submarine - though boiler plate is far closer to the ideal material there.)
Now I'm curious how boiler plate is made. Can't find much - its all code stuff per google search. Seems like its just hot rolled steel plate to a certain thickness.
From vague memory, the two huge issues for armor plate (especially ~150 years ago) are:
- Having the extremely heavy equipment needed to roll very thick plates. ~Nothing except warship armor was anything resembling that thick, for such equipment to even have been developed previously.
- The specialized metallurgy & treatments needed to make "hard" armor. Iron/steel plate for other purposes (boilers, etc.) was optimized for physical properties which bore very little resemblance to "resistant to penetration by high velocity cannon balls".
