Rowhouses are the dominant type of housing in the older sections of the Mid-Atlantic, from Alexandria, VA to New York City. In cities like Baltimore and Philadelphia, there are miles and miles of them. There are also large quantities of rowhouses in small towns like New Castle, Lancaster, Frederick, etc.
It's understandable why there aren't more rowhouses in the southern and western U.S. But why aren't there more rowhomes in New England? New England has some of the oldest cities in America. Like the mid-Atlantic, it developed an industrialized economy in the early- to mid-19th century. Boston is really the only exception. And even in Boston, they are confined to central neighborhoods like Beacon Hill.
In terms of their development and longevity, it's dictated by economics (both for owners and dwellers). You suggest that there is a North South divide for row houses, but I'd argue that it's more of an East West divide. The most populous US cities were ports with populations that relied less on land cultivation than on shipping, fishing, trade, transportation and manufacturing. Those port cities all had row houses in the colonial and postcolonial era. You mention Boston which is important because like Baltimore, NYC and Philadelphia it was crowded with mixed social and economic conditions. Old coastal towns in Massachusetts outside of Boston also had row houses and houses that are placed so close together that they could be considered row homes (Marblehead for example). It is also important to note that many row houses simply did not survive to the modern era because they were made of wood, because of fires that devastated many cities, because property was gifted, lost or sold across generations or because of urban development, modernization and commercialism.
I think that you are mistaken about Boston. There are many row houses in the Back Bay, South End, North End and Charleston. Triple deckers are more of a Cambridge and Somerville design, and most of those are post 1880.
Keep in mind that Lancaster might be on the smaller side now, but the row housing was built at the same time that the back bay was filled.
Boston at some point switched to stacking families atop one another in freestanding buildings called "triple deckers" or "three deckers." Boston also had an early and extensive public transportation system, so the development of streetcar suburbs may be a factor.
Or maybe all the rowhouses were in the West End and bulldozed in redevelopment.