Socialism

And the respective political systems of these two countries obviously play no role in this.

Do you think this is because the leadership is magically nicer for some reason, or because their political/economic system is better able to stay out of conflict?

It also overlooks that China has historically been conquered by outside forces as its leadership grew weak. And the Communist party was having enough trouble internally feeding its people and causing issues like the "break four olds" period that there was little ability to wage effective war.

Let's also not forget the the communists literally aided the Japanese in the rape of Nanking as it helped to undermine the GuoMinDang.