Why Exynos is not used by Samsung in the U.S market ?
As background, due to a patent-related agreement between Qualcomm (holder of many patents) and Samsung, Samsung cannot sell it's Exynos chipset to other phone manufacturers:
Qualcomm Might Have Prevented Samsung From Selling Its Exynos Chipsets to Other Phone Manufacturers
According to The Korea Economic Daily, Qualcomm abused the ‘standard essential patent’ license to prevent Samsung from selling its modems and integrated chipsets for around 25 years. According to the source, this is considered as an unfair practice and has been confirmed by the Fair Trade Commission.
To recap, the Fair Trade Commission had decided to fine Qualcomm $865 million over antitrust violations, which stated that the company violated the law of competition, but the chipset manufacturer intends to appeal the decision. 