Why did mass decolonization occur in africa following world war ii?

The Imperial nations could no longer afford to maintain their colonial nations so they freed them. Some had to free them because they had been promised freedom if they fought in WW2. Some had to fight for their independence, such as Israel, but they did get their independence.
The US highly frowned upon imperialism since it was the cause of WW2. They requested the Imperialist nations to give up their colonies unless the colonies refused to be freed, such as the Falklands and other places who could not function without their imperial "master".
Nationalism was the going trend among the nations. They liked the democratic self-rule and economies. The Eastern Bloc of Europe took nearly fifty years to win back their freedom.

The one that considered as a direct result of the presidential committee on civil rights’