Someone told me this a while ago and I don't know if it's true, but this is what I heard:
During WWII, U.S. candy makers had to stop using fresh milk when making milk chocolate. Instead they had to use milk powder, which gave the chocolate a 'cheesy' taste. After the war, they started using fresh milk again, but people were used to the 'cheesy' taste and wanted the milk powder-based chocolate back. Milk powder is still used in the U.S. today, so that is why U.S. chocolate tastes different from European chocolate, which is made using fresh milk.

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