Why are labor unions more prevalent in New England than they are south of the Mason- Dixon line? Has anyone else noticed this ?

It's a cultural thing.

NE and the Midwest have been homes to industry much longer than the Souht and Southwestern US. They've also been home to unions much longer. As a result, states have differing laws on unions. In the NE and Midwest, one can be made to join a union as a condition of employment. Here in the South, that's not possible ("right to work" states).

As a result, unions are weaker here in the southern US.