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Because American railroads went west. From 1860 forward all the money and expansion was being made in the flat land between the Appalachians and the Rockies, from 1880 forward between the Pacific and the Mississippi River. Most of these places were just territories, not even full states yet. Roads (if they existed) had no bridges, and the few tunnels made were deliberately overbuilt to allow for proper ventilation of steam trains. On that topic, huge steam locos could be freely used as could dome cars rather than smaller locomotives hooked up in tandem like on the east coast and Europe.
There just weren't any cities or existing development to deal with, and by the time the development came people had to deal with the railroad, not the railroad having to deal with people. For example LA's Alameda Corridor was completed done at ground level without crossing gates, remaining that way until the Alameda Trench was built in 1998. Therefore there was no reason to adopt a small loading gauge like Europe did.
