I think the .380 G42 was developed as an engineering stepping stone for the 9mm. .380 is 9mm in diameter and is known in foreign countries as 9mm "Kurz" which is "Short" in German (correct me if I'm wrong). After making an operational .380 it was probably easier to move up to a higher power cartridge just by adjusting mass and springs rather than the entire gun. So basically it seems to me that it was just a good engineering decision.

Still very happy with my S&W Shield but I am interested in shooting the 43 once the bugs are worked out