It's sour because in the US, particularly in San Francisco, it's hard to buy good bread. About 75% of the decent bread in my grocery store, both fresh baked and industrial, is sourdough. Consumers think sourdough is shorthand for quality. It's not. In fact, sourdough is seldom the appropriate bread for a meal. It makes lousy sandwiches, lousy breakfast, it clashes with cheese. It's good with creamy soups, and it's good plain with butter. But the premium bakeries all push sourdough, and so sourdough becomes synonymous with "good", when it's not.
So, serious eaters: What do you think of sourdough? Personally, I side with Minar.