It's a point to consider that the 757 was never initially seen as a US transcontinental aircraft (let alone a transatlantic one), and all the early production operated on significantly shorter routes. Looking at an early 1990s timetable here, both transcontinental, and indeed shorter trunk routes like LA to Chicago, were the province of 767s, DC10s and Tristars, and were seen as natural widebody routes. The 757 got slowly redeployed to these routes in its middle age, having been seen for quite some years as just a 727 replacement.
The routes it started out on have now come down in turn to 737s and A320s.