Why did the attempts to keep Africa colonized after World War II fail?

There are several reasons for the decolonisation of Africa in the post WW2 world - some of them indicate a decision to withdraw rather than a failure to continue. These reasons are in no particular order and are what I consider to be major contributing factors - for example it's also likely that desire for social reform within the UK itself was another factor as things like the National Health Service are not cheap to create or maintain.

This map depicts British territories in Africa and the year in which they were granted independence.

Economics

As mentioned by Jwenting, the damaged British economy was one of the major reasons for decolonisation.

At the end of WW2 Great Britain was essentially bankrupt having expended a great deal of money fighting WW2 and then being needing to spend yet more money rebuilding and repairing. Equally, Great Britain’s manufacturing capability had be massively re-geared to cope with fighting the war and so when the lend lease scheme ended a large economic gulf was left (BBC News). Great Britain narrowly avoided insolvency in 1946 when a loan of $4.33 Billion from the USA & Canada - Approx. $53 Billion in 2012 money. (The Twentieth Century, The Oxford History of the British Empire Volume IV, Brown. J. 1998)

This forced GB to adopt a policy of peaceful disengagement from the empire, handing control to non-communist governments when one was available. This meant that the UK was kept from the kind of costly actions fought by France and Portugal to keep control of their empires. (The Dynamics of Global Dominance, European Overseas Empires 1415–1980. Abernethy, D.)

The Suez Crisis

The impact of the Suez Crisis is largely down to economics and American Foreign Policy (Again mentioned below).

In July 1956 Egypt nationalised the Suez Canal, a vital link for world shipping. Prime Minister Anthony Eden and the government of France engineered a situation whereby Israel attack Egypt in response. This then allowed the UK and France to intervene and take control of the Suez Canal. (The Rise and Fall of the British Empire. Lawrence, J).

In January 1956, Guy Mollet was elected prime minister in France and promised to bring peace to Algeria, a French colony, in the throes of a nationalist uprising. But the presence of a million French settlers there made a withdrawal from Algeria politically impossible and his attempts to resolve the situation escalated the violence.

Meanwhile, Israel, greatly concerned about Egypt’s rearmament and involved in a series of border clashes with Egypt, was purchasing aircraft and weapons from France. The French government had been meeting secretly with Israel and invited Britain to join the negotiations.

In October 1956, Mollet, Eden and Israeli Prime Minister David Ben-Gurion met at Sevres near Paris and concluded a secret agreement that Israel should attack Egypt, thereby providing a pretext for an Anglo-French invasion of Suez.

BBC History

However, this move was not backed with the support of the United States, and prompted President Eisenhower to threaten to sell US reserve of the Pound Sterling. (Empire. Ferguson, N.) This would have caused economic ruin for the UK and forced the UK to withdraw from what was otherwise a militarily successful operation.

This forced withdrawal showed that the UK could no-longer act without approval of the United States, and following the resignation of Prime Minister Eden caused his successor, Harold Macmillian to accelerate decolonisation in order to help win back favour from the United States. (The Oxford History of the British Empire, Vol. 4: The Twentieth Century, Brown J. M, Lewis W. R.) The fact that the UK still enjoys a “special relationship” with the US where as France’s relationship with the US was severely diminished following the crises backs this up so some degree. The Suez Crisis also marks a turning point in British foreign policy with Great Britain moving away from being a great colonial power.

The Atlantic Charter

In August 1941 Prime Minister Winston Churchill and President Franklin D. Roosevelt issued a statement setting out the Allies aims concerning the war, and the post-war world. While this was only a statement, and there was not a signed copy, agreement to the terms of the charter was telegraphed by the British War Office to the United States and vice versa.

Further more, the governments of Belgium, Czechoslovakia, Greece, Luxembourg, the Netherlands, Norway, Poland, the Soviet Union, Yugoslavia, Free French Forces agreed to abide by the terms of the charter at a meeting of the Inter-Allied Council on the 24th Sept 1941.(Source)

While the entire text of the charter can be read by clicking the above link, part 3 is most relevant to decolonisation

Third, they respect the right of all peoples to choose the form of Government under which they will live; and they wish to see sovereign rights and self-government restored to those who have been forcibly deprived of them.

This part of the charter specified commitment to self-determination, although Churchill maintained that this only applied to Axis occupied territories, which gave a great deal of hope to independence movements within the British Empire. In fact, it prompted Mohandas Karamchand Gandhi to write the following to Roosevelt

"I venture to think that the Allied declaration that the Allies are fighting to make the world safe for the freedom of the individual and for democracy sounds hollow so long as India and for that matter Africa are exploited by Great Britain..."

Mohandas Karamchand Gandhi

It is not unreasonable that revolutionary parties in Britain's African colonies came to the same conclusion as Gandhi with regards to how the Atlantic Charter should be applied.

Ultimately, Britain's withdrawal from it's empire was forced, but largely free of conflict within Africa (although, there were exceptions). Personally, I think the reasons for the French decolonisation of Africa needs another question.