I imagine it's the same reason why many of them make a big deal about the gowns worn on "the red carpet" during awards shows. Women strive to be unique when it comes to clothes in general. However for many of them a wedding is their one day to dress up like a "fairytale princess" in real life. Some of them have been dreaming about this moment from the time they were little girls.
Truth be told the groom is slightly more than a prop at most weddings. The wedding day belongs to the woman. Everyone in the audience stands when (she) walks down the aisle.