The marriage of Queen Victoria to her cousin Albert of Saxe- Coburg in 1840 has had more influence on weddings than any other. Queen Victoria put the wheels in motion by marrying in white. Though brides continued to wed in gowns of different colors, white was now set as the color of choice for weddings and has continued ever since
Queen Victoria chose white NOT because it symbolized purity, but because she wanted to show her people that she would run the country in an economical way. White was a much less expensive color to make than the colors typically popular for wedding dresses at the time (red, black, and purple). Plus, it gave her the option of using some lace that she already owned, rather than having something specially made.
So basically, the white dress was originally a political move.