Pistol cartridges had a trend of getting smaller, lighter, and faster hence the trajectory from .45 acp to 9x19mm and now 5.7mm. The invention of the personal shield (calling it armor is a misnomer in my opinion but both terms are used interchangeably here) in Strinova reversed the trajectory of pistol development.
Shot placement became irrelevant when most of the damage would be absorbed. Large volumes of fire or enough momentum to carry through the armor was needed. Smaller caliber, semi-automatic pistols were deemed obsolete for its low volume of accurate fire even with increased capacity and low recoil especially in the police context. The lightest caliber we found that made shot placement relevant again was the 50 AE of the Athena. The pistol to fire it was originally called the Desert Eagle, but for some reason it became the “Athena”. I am guessing it is so manufacturers can supply the PUS given the legal limbo of the design during the collapse.
The question now is why not create a different, better pistol and cartridge? My educated guess is that the introduction of personal shields was just that abrupt. As mentioned before, it reversed the previous trajectory of pistol development and it seemed it was that abrupt that they needed a readily available stop-gap hence the choice of 50 AE. As for the gun, it is a similar reason for the Desert Eagle was the most common hand gun in strinova to chamber the round because popculture. They found it more expedient to legally skirt around the legalities than create a new one.