Because historically, the companies that produced the best chocolate ingredients refused to sell them to American chocolate making companies. The extra sugar covers up the taste of the not-as-good cocoa. I recall a big thing in the news when someone merged with (I think it was) Nestle because, as a non-American company, they had access to this better quality cocoa, and there was concern that it would be an unbeatable competitive advantage. I believe the companies decided enough already and it was okay to sell to Americans, but the “standard" for what chocolate should taste like was already set.