I'll repeat a postulate I saw on another forum. The x39 cases and most/all other cartridge cases originally designed to be manufactured from steel are tapered to provide optimal feeding and extraction from the chambers of battle rifles. Using this design as is by simple neck reduction is counter productive from an accuracy and performance standpoint when using brass cases.
Chamber pressures can be higher with parallel case walls(brass cases) w/o excessive bolt thrust. Performance and accuracy are normally enhanced since there's less bullet to neck misalignment with less tapered sidewalls.
The less developed countries used what was available w/o much regard for what might have been improved as witnessed by the decades long use of the 7.62x54/ 7.62x39 when better, more powerfull, modern designs could be had. What was good enough in the 1890's or 1947, was/is still good enough.