It's a cultural thing. Just like why are most Congregationalists and Episcopalians Republican? The answer is that they historically stuck to the GOP back in the days when that party represented moderate-to-liberal New England upper crust WASPism.
Nowadays, however, that's changed, as those are among the denominations whose polities are becoming more associated with the political left.
The members of those churches, most likely, are far more right-wing than the clergy and doctrine espoused by both though, so it's hard to say how Democratic religious WASPs really are at this point.
That really isn't the case. I'm not positive about Congregationalism but the divisions of the Episcopal Church telegraph pretty closely at most levels of its structure, by nature of its polity. Right-wing parishes and dioceses aren't going to be choosing these people as their rectors and bishops. There's a definitive progressive, often left-leaning-Anglo-Catholic or 'emergent' depending on the parish majority, and a sizable conservative, often Evangelical-leaning minority, in the House of Deputies as well as the House of Bishops.
I've just noticed that that's the case for mainline Protestantism in general. Active members of PCUSA and ELCA congregations are right-leaning, even though the clergy of both voted strongly for Gore. The PCUSA, in particular, has a democratic structure and at the General Assembly, it was decided that gays can become pastors, that Arizona was to be boycotted because of their immigration policy etc.
Yet look at these figures:
Note to BRTD: I'm speaking of how Democratic
active Episcopalian and Congregationalist members are today. I think it would be difficult to pinpoint how Democratic they are because as a demographic, active church goers of mainline protestant congregations tend to be much older and slightly more conservative than younger members that casually affiliate with them (I'm partially speaking from my own experience, although data backs me up here). I doubt that they're a Republican constituency but I could see them being swingy group that's in the political middle.