Because people back in old times old Men would marry very young virgins but they would have all ready been unpure by other people that there father hired or rented gross right. Also by the time they already married he could get tired of her a pick a new one.
Historically, white symbolized wealth. When Queen Victoria wore a white wedding gown, she was doing so in a time when most women could not afford a dress they would only wear once, let alone a WHITE dress which was harder to keep clean, given the technology of the time. So as a result, women frequently got married in their best dress,Celebrity Dresses
, no matter what the color, as they would need something they could wear again.
Blue has been recognized as the color of purity for much longer. Simply do a search for "Origin of white wedding dress" and a number of articles will pop up explaining this.
I don't know quite when a white gown was ever supposed to symbolize purity. That is a rather new idea in the grand scheme thi