TLKY,
The Gemara in Kesubos indicates that the honor due a Kallah is one step below that of a King and a King was praised by the Chachomim for giving honor to a Kallah. It is hardly a stretch for Klal Yisroel to choose to show this honor by standing when she enters the room.
Furthermore there is an express mitzvah in Chazal to be m’sameach a Kallah. Is it not reasonable to think that a Kallah might feel a little simcha when everyone rises for her when she walks down the aisle? And shouldn’t those sitting down (and especially if everyone were to sit down) be worried that by doing so they might lessen the Kallah’s simcha?