"principal street of a (U.S.) town," 1810, from main (adj.) + street. Used allusively to indicate "mediocrity, small-town materialism" from late 19c., a sense reinforced by the publication of Sinclair Lewis's novel "Main Street" (1920).
But a village in a country which is taking pains to become altogether standardized and pure, which aspires to succeed Victorian England as the chief mediocrity of the world, is no longer merely provincial, no longer downy and restful in its leaf-shadowed ignorance. It is a force seeking to dominate the earth, to drain the hills and sea of color, to set Dante at boosting Gopher Prairie, and to dress the high gods in Klassy Kollege Klothes. Sure of itself, it bullies other civilizations, as a traveling salesman in a brown derby conquers the wisdom of China and tacks advertisements of cigarettes over arches for centuries dedicate to the sayings of Confucius. ["Main Street"]