Irrespective of the 400-years or so colonialism of the Americas or the Philippines, how is it that 19th century European nations could've decisively used her massively technologically-superior militaries to smash the still independent countries of Africa and Asia in the span of months or a few years, only for the Europeans to seemingly pull out so abruptly out of these conquered nations just 60+ years later, whereas it took 300+ years for the conquered areas of the first wave of European colonialism (15th-18th centuries) to declare independence?
The main answer here is WW2. If we look at the predominant colonial powers at the time such as the Netherlands, France and the United Kingdom we can see that they were either occupied by a foreign power or severely damaged by it. The war crippled nearly every aspect of these countries economies and made holding on to areas inhabited by subject people extremely unfeasible economically. Also, the result of ww2 was a transfer balance away from the colonial powers and to the new superpowers, the USA and the USSR.
The USA made it clear to its allies that they would not support a continuation of imperialism, and as these nations were extremely dependent on US aid such as the Marshall plan, it was the vital they followed this. The old powers did make attempts to reassert there dominance, such as the Suez Canal crisis, but this led to a negative response from the US. Portugal is a colonial power that was neutral in the war and was able to hold on to Mozambique and Angola until the 70s, which led to a bloody struggle with rebels supported by the USSR and other communist states, before the fall of the Estado Nova regime led to the independence of those nations. The USSR and other communist states also pushed Marxism as an ideology of liberation for the oppressed people in these nations, making colonies extremely difficult to hold onto.
Overall, its a combination of these factors. The weakening of the predominant colonial powers in WW2 made it extremely difficult to hold onto the colonies, along with the power shift to two superpowers who opposed colonialism for the own reasons further helped the colonies to gain independence. Also most of the colonies were exploitative in nature. This means that whereas the countries settled in the first European wave of colonisation were often settler colonies, composed of descendants of the colonizers, these newer colonies were comprised of the indeginious populations furthering the difficulties of holding onto them after the war.
With such a large and diverse area covered, it is very difficult to narrow down all the reasons and this is the roughest outline of why, I hope it makes sense! :)