Besides matters of taste or tradition, a royal wedding is also often an Act of State, with the presence of Government, Parliament, "High Colleges of State", the judiciary, the armed forces, the Corps Diplomatique, etc. Not to mention heads of state, crowned heads and other distinghuised guests. It is common sense to dress conservatively, befitting the solemn event.
Besides that, such a wedding is the first event of a royal bride (when marrying into a royal family) to "set" herself. A Letizia entered the church, Her Royal Highness The Princess of Asturias left the church. A Mßxima entered the church, Her Royal Highness The Princess of Orange left the church. A 'girlie" or a "sexy" look does not combine very well with the dignified, solemn, regal and stately appearance a new royal wants to make. A 'girlie' or a 'sexy' look also does not combine very well with the often historic jewels or an antique veil which is combined with the gown.