The harlot of Revelation 17 is dressed like a priest - robes of blue and scarlet, precious stones, an inscription on her head. So is the bride of Revelation 21: She is a city adorned with precious stones with streets of gold.
Why would a female city be dressed like a priest? Because both cities are priestly cities - the old Jerusalem named Babylon, and the new Jerusalem that comes out of heaven.
But we might, as one of my students, Fraser Martens, suggested, ask the question inside out: Why would a priest be dressed like a bride? And the answer is that the priest represents the bride in her approach to Israel, offering bridal food on the altar, doing the house-cleaning, offering her advice to Yahweh in prayer. Every sacrificial feast is a marriage supper.