What factors led to the collapse of European colonial empires after World War 2?

Answer 1

This explanation written out on Wikipedia's article called The British Empire. It shows why Britain and other countries began to de-colonize.

***Decolonisation and decline (1945-1997)

Though Britain and the Empire emerged victorious from the Second World War, the effects of the conflict were profound, both at home and abroad. Much of Europe, a continent that had dominated the world for several centuries, was now literally in ruins, and host to the armies of the United States and the Soviet Union, to whom the balance of global power had now shifted.[127] Britain itself was left virtually bankrupt, with insolvency only averted in 1946 after the negotiation of a $3.5 billion loan from the United States,[128] the last instalment of which was repaid in 2006.[129]

At the same time, anti-colonial movements were on the rise in the colonies of European nations. The situation was complicated further by the increasing Cold War rivalry of the United States and the Soviet Union, both nations opposed to the European colonialism of old, though American anti-Communism prevailed over anti-Imperialism, which led the US to support the continued existence of the British Empire.[130]

The "wind of change" ultimately meant that the British Empire's days were numbered, and on the whole, Britain adopted a policy of peaceful disengagement from its colonies once stable, non-Communist governments were available to transfer power to, in contrast to other European powers like France or Portugal,[131] which waged costly and ultimately unsuccessful wars to keep their empires intact. Between 1945 and 1965, the number of people under British rule outside the UK itself fell from 700 million to five million, three million of whom were in Hong Kong.[130]