|I'm sorry last time I checked you were NOT Pippa Middleton|
Because its such a well known rule, it makes it that much of an offense to the bride. Since everyone knows not to wear white, the bride will probably take it as a jab to her if you do so. Also, everyone will be talking about it and therefore taking attention away from the bride.
As I said, there is always one in every crowd, but why does this person become the person the bride has a problem with? Well it does make for a good story. At my friend's wedding it was one of the groom’s guest (a mother of a highschool gal friend, not ex-girlfriend, that's a different post). Having friends of the opposite sex at your wedding could already be a sensitive issue so of course that makes this particular woman's clothing choice even more insulting. Whether you mean to or not, by wearing white to a wedding you are drawing attention to yourself and away from the bride.
Just stop wearing white!
This goes for men too. I recently had someone tell me they were at a wedding and a male guest was wearing an all white suite. What? Who even owns one? Bit over the top for a wedding, don’t you