Rhoticity distribution in the US has both historical and social reasons. The first-found colonies in the North (New Hampshire, Mass., Connecticut, Rhode Island) and South (Maryland, Georgia, N&S Carolina, Virginia) were non-rhotic right from the start, because the English who settled there came from regions which at that time were already non-rhotic (esp. London and Middle-England). Moreover, they stayed in touch with they home country so that their non-rhotic accent was kept alive.
The Midland American English Founder colonies, however (Pennsylanvia, Delaware, NY, New Jersey) was settled by nonstandard-speaking people from Northengland, Scotland and Ireland, thus a rhotic region separated non-rhotic southern and northern founder states.
These rhotic states played a major role during the westward movement, so that most of the US became rhotic and has remained so until nowadays.
Yet for a fact, rhoticity is viewed differently in different parts of the US: There are still some relic areas in the South in which postvocalic [r] is socially marked. In non-rhotic New York, however, rhoticity has gained prestige and is increasingly used by young and upwardly mobile people (LABOV 1966).