Basic answer: Over expansion
Germany had been deprived of its colonies at the end of WW1 and the vast number of islands the Japanese had captured had been liberated by the US. This led to these places gaining their independence. The main reason though was the British Empire which in a way dissolved. No longer did the sun never set on the British Empire. Basically with the prime example being India The British realized that in a modern world there were other world powers and they had too much land to protect. They had troops in every colony from Atlantic to Pacific and simply couldn’t defend their whole country. This whole idea was brought out by the Second World War and thus after the war many countries gained their independence.