[She gives this some thought.]

... It's one of those traditions that's been done for so long that people have forgotten just why it's done. It's just thought of as the right thing to do. But everyone in my kingdom knows what will happen when a man gets down on one knee. Even just seeing them go down is enough to make a girl gasp.

If I had to guess why... I'd say it's like a knight vowing to protect his precious princess!