Both the First and Second World Wars were economically crippling, but victory in 1945 came at a much higher political cost for the European empires than it did 1918. In the Second World War, the Allies were routed before they consolidated their strength and beat back the Axis Powers. The French were decisively defeated on home territory in just a month, while the British were humiliated by the Japanese at Hong Kong and Singapore. This shattered the myth of imperial invincibility that cemented European supremacy abroad. Even after victory, the damage was done and the credibility of empire could never be restored.

I think another dimension of it is that after defeating the imperialism of Nazi Germany and Japan in the name of liberal democracy, Britain and France didn't really have a leg to stand on with regard to their own empires. I mean, the Atlantic Charter, which established the war goals of the US and UK in 1941, included the commitment to uphold "the right of all peoples to choose the form of government under which they will live". That's pretty hard to square with colonialism, no matter how you spin it.

It is of course worth remembering that the empires of Europe didn't fall apart immediately in the wake of the Second World War. India achieved independence in 1947 but most of Britain's African colonies weren't released until the 1960s. France fought two particularly bloody colonial wars to hang on to their territories in Indochina and Algeria. Portugal was still fighting to keep its empire in Africa into the mid 1970s. The Second World War didn't lead to the direct collapse of European empire, but it did undermine its base of legitimacy.

I suppose ultimately what distinguished the Second World War was that it gave birth to new political expectations among colonised people which didn't really materialise after the First World War, despite Wilson's rhetoric of self-determination. Nationalist movements within European empires were more fully formed and better organised in 1945 than they were in 1918 so they were in a stronger position to take advantage of the post-war political moment to assert their rights. Of course, this subject is the basis of a whole field of study so I'm sure I've oversimplified things to an extent.