Are you genuinely asking why Sophie is not a Princess? In England women who marry Princes do not automatically become a Princess (one exception was Princess Alice, Duchess of Gloucester who was made a Princess on the death of her husband, the Duke of Gloucester, mainly to differentiate between herself the new Duchess of Gloucester.) If the man they marry has a Royal title (Duke, Earl etc.) they then take the highest female title equivalent (Duchess, Countess etc.). If, however, their husband has no Royal title other than Prince, then they become Princess "Insert Husband's Name Here" (e.g Prince Michael of Kent's wife is called Princess Michael.) Sophie is officially The Princess Edward, but uses the female title of her husband's highest title, Earl of Wessex.
If you were not genuinely asking why Sophie was not a Princess, I now look like a right fool, haha.