In addition to the point about Western mythologies dominating because of cultural exports, I think there is also the undercurrent of England’s original mythologies having been “lost” and so the English were always fascinated by the mythologies of the Norse (due to being invaded) and by the Greeks and Romans (as previous “great” civilizations they aspired to be).

Combine that with America’s obvious English influences and the influence of England as a colonizer around the world, and those mythologies gained a huge outsized influence.