The colonizing countries couldn't afford to maintain a presence in the colonized countries. Running a colony was very expensive, and the government didn't always see a strong return on their investment. The society, maybe, the industrialists and traders, certainly, but not the government itself, any more than opening up a new mine, or building a factory gives the government money. After the second world war, the colonialist countries for the most part couldn't afford it. Sometimes it was easier to just roll back the clock a bit and go back to trading via proxies rather than direct control. Remember that what we think of as colonization was only a very brief period, all things considered, and before that, the relationship tended to be a lot more indirect. Direct colonization often turned up where internal problems demanded intervention or the colonized country was weak or unstable enough that extracting resources or securing settlements demanded direct intervention.
Secondly, the colonizers were too weak to hold onto the colonized countries due to economic and military exhaustion.
Thirdly, the cultures of the colonizing countries were changing, making it less likely that anyone would support the imposition of their will on a foreign country in such away.
Fourthly, the colonized areas had developed independence movements that were sufficiently powerful and organized that they could take action.
Fifthly, the Cold War tended to involve both sides sniping at whatever geopolitical weaknesses existed on the other side. Colonies or dependant nations were easy targets for propaganda and espionage, indirectly weakening a given bloc.
