Keep in mind the US immediately slashed defense budgets and demobilized. Obviously, weapons development didnt just stop- especially with h-bombs, rockets and jets- but the ground forces were in pretty abysmal shape by Korea.

5 years passed between the abolishment of the office of civil defense and the FCDA. Factoring in this slow start to the cold war and the M14 initiative, the development of the AR15 lagging so many years behind the AK and STG makes more sense.

The AKM didnt enter production until 59. Heck, Berlin wasnt sealed until 61. The Chinese and North Koreans were still using burp guns and conventional rifles, not assault rifles.