And just because Hershey's is the best-selling chocolate in the US doesn't mean it's Americans' "favorite." If you gave Americans a Lindt truffle and a Hershey's bar in a side-by-side blind taste test, I'm pretty sure the truffle will win pretty handily.
French and German mustards have strong, specific tastes. Without a shared cultural heritage to help enforce those tastes through generations, blander, more widely palatable alternatives were developed.