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Before the widespread industrialization of the dairy industry, most butter in the United States was made with cultured (also called clabbered) cream. Fresh cream from the evening milking would be allowed to sit out overnight so that the milk could settle and the cream could rise to the top.

During those unrefrigerated hours, the micro-organisms in the milk caused it to sour ever so slightly, taking on a nutty tanginess. This cream was then churned into butter, which retained those delightful flavors.

Once all dairy was routinely pasteurized the active cultures were killed, and therefore the milk could no longer sour on its own. Thus it became easier for farmers to make butter out of sweet cream, because creating cultured cream added another step (live cultures would have to be added back into the pasteurized milk, which is now standard practice in Europe).

This country grew used to the milder taste of sweet cream butter. That is, until it rediscovered cultured butter via the imported European stuff back in the 1990s. Because of its growing popularity, a handful of American dairy farmers started producing European-style high-fat cultured butter.

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– http://www.nytimes.com/2013/10/09/dining/making-cultured-butter-at-home.html?ref=todayspaper