I think Hershey puts out some quality products. Nestle owns so many different "brands" (like Dove I think) who are also quality chocolate. I think it's mostly what you're used to, what's easily accessible, and THEN personal taste buds. If you grew up with American chocolate, given at every holiday, ever, you notice the differences (however small) foreign chocolates have.
I think other foods also play into this. How many Americans' favorite food is American style cusine? I know I prefer the various non-spicy Asian foods as well as Italian over meatloaf and mashed potatoes.