The US and most of its allies went all in on the battle rifle concept based on their experiences in WW2, especially the success of the M1 Garand. This led to the G3, FAL, and M14. The Soviet experience led them to the assault rifle concept. It’s important to note that the Eastern front was larger and had a lot more combat than any other theater of the war.

Weapons in the Korean War were primarily WW2 designs, so it’s not surprising that the US didn’t realize the superiority of assault rifles until the M14 faced the AK47 in Vietnam