Theodore Dalrymple has written a nice piece for Takimag.com titled "Slobbery as Snobbery."
As you probably surmised from the title, it's not only about the pervasiveness of slobwear today, but theorizes that a big part of the cause is reverse-snobbery, the mentality that no one should rise above anyone else, that dressing elegantly is "elitist," "classicist" and "undemocratic" (my terms, not necessarily Mr. Dalrymple's).
I was in a restaurant on Father's Day, and probably 75% of the men I saw were wearing basketball-type shorts with either dirty tennis shoes or flip-flops/sandals, and with either intact t-shirts or "wife-beaters" (t-shirts with the sleeves cut off, which look even worse than tank-tops). I don't understand what's happening in our culture. As Mr. Dalrymple notes, the issue isn't people dressing casually per se, as that can still look appealing; it's that people today seem to be deliberately dressing in the ugliest clothes possible, like they're intentionally trying to become eyesores, to create the maximum possible aesthetic offense to the other people who have to look at them.
And, as if that weren't bad enough for a regular day, Father's Day is supposed to be a special day; we seem as a culture to not only be losing the mentality that we should dress in part to be visually pleasing to others as a matter of course, but that dress should reflect the occasion, that we should dress differently to take dad out for his annual special day than we do to mow the lawn.
I suppose the good news is that it's hard to imagine things getting any worse. Have we bottomed out yet?