People from all three eastern regions (new england, southeastern, midlands) migrated into the midwest, great plains and western united states. why is there a relatively uniform form of english (dialect) spoken across this enormous area?

This is because in the "U. S. A.", people in all the regions in the U. S. A. historically migrated throughout an "entire land mass" (essentially); and this "land mass" tends to be more "homogeneous" (as opposed to "British English" speakers; who speak with different dialects, accents). Note that the United Kingdom, including Great Britain, is further isolated from the U. S. A.-by a huge ocean - than the "relatively more homogeneous physical region" / and thus the "relatively more linguistic region".