Question

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?

Answer 1

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".