It's true that the population is shifting to the south (both east and west). Before I say anything that may get me flamed, I'd like to say that I have family in Massachusetts and ancestors that fought alongside the original GW. THat being said.......
In the early/mid 1800's, the south had more fertile farmland, more pleasant weather, and greater opportunities to be self sufficient. We were producing goods at a lower cost and better quality than the northern states. When the northern economy starting going south (pun intended), the politicians who were desperate to keep their electorate happy, started making an issue of how unfair the south was, what with cheap/free labor in the slaves and indentured servitude. Part of their strategy was to promote anything that would be opposite the south's interests. And since the south was predominantly white protestant with a very strong sense of religion and morality, the northern politicians started promoting ideas and agendas that were designed to enflame southern attitudes.
Unfortunately, that mindset has seemed to become engrained in the culture of the north, at least publicly.
I honestly don't believe that the average, everyday northerner is any more liberal/conservative than any average, everyday southerner. I thinik it is the people/politicians who make the best newscopy get the spotlight and that is what the majority of people base their opinions on.