Likely a combination of generational changes (similar to how The South went from being part of The New Deal constituency to the conservative stronghold it is today), as well as a shift from social conservatism to a more social gospel type of Christianity during the 19th century, revolving around the abolition of slavery.
Anglicanism is historically way more conservative than Puritanism.
I was just about to say this (PR linked me to this thread, hence the necro). Even today the UCC, the successor denomination of New England Puritanism, is even more liberal than the Episcopal Church. Certainly no abolitionist in 185whatever would have converted
from a Nonconformist-derived denomination to Anglicanism.
