It's kind of interesting because it's hard to pin down. I used to think it was a liberal/conservative thing (back when my point of comparison was mostly just rural Lutheran churches only olds go to and more modern family-oriented ones like where my parents go to now), but I've seen in videos of megachurches and even Assembly of God services and on their websites most people are wearing jeans. Similarly when I was returning this morning I passed a church near me which is one of the most blatantly liberal ones in the state (to where they are quite active in opposing the anti-gay marriage amendment and hosting speakers against it) and noticed people leaving, most of whom were somewhat formally dressed and a group of olds crossing the street in front of me who were all super-formal and the men had ties and everything. And they are way more theologically liberal than most churches to to the point of not preaching belief in a physical Resurrection, which no one would do at where I went. Yet they also host another smaller church in the afternoon that is just as liberal politically but theologically more conservative that I've had some involvement with, and no one there dresses up. And if you look up an Episcopal cathedral on YouTube you'll see they tend formal, but so do Mormons and I'm sure that a LCMS church would be way more formal than any ELCA one, and as I mentioned a few weeks ago most people were pretty dressed up at my cousin's first communion in a Catholic church in a conservative suburb, so you can't say it's correlated with theologically liberalism either.
So what is the correlation? It almost seems arbitrary in regards to political and/or theological liberalism/conservatism.
I think traditionally it has more to do with people doing menial labor, working long hours and church being one of the few, if only, opportunity every week to interact with others in their community at a more personal level, and they therefore felt the need to be more presentable.
I guess one of the reasons church attendance is so low in societies with a high percentage of white collar workers is because they have to look their best every morning when they go to work and what the church offered them as members of the community simply doesn't have the same appeal anymore.
