How did New England become so secular and socially liberal?

[Bleach Blonde Bad Built Butch Bodies for Biden]

New England (particularly southern New England) was historically the epicenter for Puritanism and American ultra-conservative/fundamentalist Christianity.  While it's been argued that the modern Religious Right was born in the American South out of a mixture of Scotch-Irish and slave owner culture, the Puritans, who had wanted to establish a theocratic society, died out as a religious movement, and the only prevailing "liberal" or "progressive" attitudes of their time were opposition to westward expansion, opposition to dueling, and the valuing of education.  Although New England states historically had stricter abortion laws pre-Roe than the more conservative Southern states, they eventually came to adopt lighter abortion restrictions, disliked Prohibition, and were among the first states to embrace same-sex marriage.

The old Congregational churches, in which the Puritans had worshiped, are now completely unrecognizable from the churches of their time period and they are now under the domain of the uber-liberal UCC.  From my understanding, this could only mean that somewhere down the line, a major theological shift occurred among the Congregationalists in order for them to have shed their Puritan ways.  The influx of Irish and Italian immigrants to New England were Catholic, so they couldn't have had an impact on the changing norms of the Congregational churches or the secularization of society at large.

So what happened, exactly?  What caused those historically puritanical churches to later adopt a liberal theology and how did New England come to be the least religious region of the United States?  And why and how were these progressive-minded Congregationalists able to change their churches from within rather than join or start a completely new Protestant sect?

[Bleach Blonde Bad Built Butch Bodies for Biden]

In my view, the real question is how Episcopalianism became so socially liberal. For centuries in both England and America, high church Anglicanism was about as Tory and upper class as you could get. But today, Episcopalians are somehow the most socially liberal church there is. Really mind-boggling if you ask me.

As a practicing Episcopalian, this question has boggled me for some time. Insofar as "church shopping" was a thing, I would not have been an Episcopalian for most of its early days despite my Anglo-Catholic tendencies.

Lutheranism or Methodism probably would've been my next stop. Or a non-Roman Catholic Church.