Also should acknowledge that the Great Lakes and Mississippi River (and major tributaries) made for efficient water shipping to a lot of the major cities of 19th century America. Cincinnati, Chicago, Cleveland, Detroit, and Milwaukee all have ports.

And yeah, as you say, there just weren't major settlements of European Americans anywhere else yet except in the plantation south. California wound up aggressively settled before transcontinental rail, but even there was largely along the coasts. Our national population remains pretty coastal alongside density in the great lakes and major tributaries of the Mississippi.