hey there i'm not sure if i understand your question, but i'd have a guess any way
i would say
1. the need for reconstruction in the post-war era kept the european nations unable to wage military forces strong enough to be used to maintain colonies.
2. increasing tension between NATO and the USSR, along with the fact that the USSR had an army of overwhelming number forced the European nations to focus on self-defence. thus it became impossible for European nations to draw spare forces to occupy the rest of the world.
3. The destruction caused by WWII would have had severe impact on ordinary European citizens. The fear for the possibility of a nuclear war allowed pacifists to gain more recognition from the general public than ever. Old ideals such as building an empire so large upon which the sun never sets all day round now seemed hollow and impractical.
