The experiences are far more fresh in the minds of those who live in those countries. There has been little time to heal the wounds (many of which go back centuries and have had recent refreshment).
Most likely due to internal strife (usually ethnic) as a result of oppressive regimes (post invasion/colonial absorption).
It'll take time, and with sensible education of the most recent generation(s) by enlightened fresh governments, those wounds will heal.
Diplomacy is hard. Fortunately, it seems the world is becoming more in tune with harmony as nations' wealth increases. More distribution needed obviously, to prevent serious disparity in wealth that would lead to more conflict.