100 years ago I will bet that Northern New England would have beat many other regions of the country in religiousity. Why has this been almost completely reversed since then? The children of overly religious parents breaking away yet keeping there affliations to there churches. I mean there is almost nothing Puritanical about the Congregationalist or Unitarian churches anymore.
I think it reflects national trends. Mainstream Protestantism, ethnic white Catholicism, and all branches of Judaism other than ultra-Orthodox have seen big drop-offs. Evangelical Protestantism, both native and immigrant, are where you see the biggest increases and they're largely absent from New England outside of some evangelical congregations in Maine and immigrants in the Boston area. New England denominations aren't behaving much differently than they are elsewhere in the U.S., but their proportions are very different.
The influence of the Catholic Church has declined substantially because of generational change (in addition to liberalization, people move more so they're less bound to their home parishes) and because of sex abuse scandals.
Holmes... if you want to see why Catholic observance in Maine, New Hampshire, and Rhode Island has dropped off... many of these congregants have cousins in Quebec, and the trends there are even more marked.