I asked one of the hosts about this once, she was concerned that "people might not come" if it was black tie required or they were too rigid etc...
That seems to be the general consensus these days. Except for the uber-rich, most Americans don't appreciate dressing up. You can hardly get the common man to wear a suit and tie for church or weddings any more. So organizers of evening events typically won't call for formal wear, or won't enforce it, lest they be accused of being snooty and discriminatory. And ye gads! Being accused of snootiness and discrimination will mean bad press, demonstrations, and loss of business! Possibly even lawsuits!

I don't know how or why America got to this point, but it's a bit of a shame.

Do common people in the UK (i.e. the middle class) have black-tie events on a regular basis?