Butyric acid, when Milton Hershey started producing chocolate his process overheated the milk causing it to undergo lipolysis.

At the time milk chocolate was rare in the US so he didn't notice that the flavor was slightly off. After Hershey's won several contracts for rations during WW2 (and I want to say WW1 as well) which caused the slightly acidic version of his chocolate to become popular.

Thus people either grew up with Hershey's chocolate (with a slightly acidic taste) or European milk chocolate (no acidic taste), and prefer the version from their childhood.