They were running out of funding.
Also it was easier to hold and fire for younger Soldier training.
Just off handed thoughts on why they would swap from the 7.62x39 which as someone mentioned, is a killer not a wound maker.
Blessings
Yup. Bigger round,more powder and a perfect aim.
The Nazis started the trend of trading full battle rifle firepower for more rounds and control offered by an intermediate cartridge.