Carter did have some trouble with the white Catholic vote. An article in the NYT from September 1976 indicated that Ford was making a play for Catholics (conceding the Southern Baptist vote to Carter):
https://www.nytimes.com/1976/09/05/archives/ford-hopes-linked-to-catholic-vote-aides-say-appeal-to-group-in-the.htmlAn anti-South and anti-evangelical sentiment must have played a role. Carter just didn't perform that strongly in the Northeast suburbs. He lost CT and NJ and Long Island (which made NY close). And as you mentioned, his performance in MA (and RI) wasn't all that great.