OK, lets do a wee bit of metalurgy. Anyone with differing information is most welcome to step right into this. I'm not nterested in being right, rather that we all know the truth.
Steel changes modulus very little due to hardening. Modulus is the amount of strain (movement/deflection) you get for a certain stress (applied force). Hardened steels simply have a higher yield point. Example: if I made two identical springs from "soft" and "hard" steel, they would behave almost identically as springs until the "soft" one was deflected beyond its yield point. The "soft" one would now be permanently bent (dimensions altered) while the hard one would return to original dimensions. Now, same scenario except we case harden the "soft" one. Again, almost identical behaviour until we strain the "soft" one past the core's yield point. At that time, the core is bent and the case is trying to return the core to original dimensions. Calculate the difference in strength between the core and case shell and you will know the amount of influence the case shell can have. Now, think of the action as the spring. Many times over the amount of metal in the core as in the case shell!! Post if you don't follow that logic or disagree.
Shotgun actions were designed and developed (cut and try) using mild steel. It was what was available, more or less easy to work, reasonably priced, and workable sections met weight and strength requirements easily. The metal industry was used to case hardening - SOP. The difference in surface wear and corrosion resistance (think heavy shooting days in damp 'ole UK with black powder and no modern oils or cleaners) between a "soft" steel surface and a case shell surface is remarkable. So, it was relativly a small jump to case harden actions. CCH was a byproduct and not even used intentionaly early on.
Clearly, a "soft" receiver with a case shell is better than a through hardened receiver that is sufficintly brittle to be prone to cracking. Case hardened "soft" receivers were plenty good, so there iwas little push to develop a through hardened receiver. Modern steel alloys and machining techniques have, of course, changed this picture.
DDA
