I am not an Imperial history expert nor have read in depth, but there is merit in Dayuhan's last post - blame anyone, especially a dead US President, for our own failures.

In 1945 Great Britain, France and the Netherlands were exhausted. Two had undergone the deep trauma of defeat and occupation.

I suspect part of the answer why people
believe what they will
lies in psychology and politics. A contemporary illustration maybe found in this UK politicians comment:
Business Secretary Vince Cable says it has been a challenge for the government to explain to the public how bad a state the economy is in.
Change a few words and maybe it would fit 1945?

Incidentally I do not agree with his explanation, mainly as it appears to ignore the impact of political decisions for many years on the UK economy.

Link:http://www.bbc.co.uk/news/uk-politics-13480971