Does anyone have any idea why Princess Grace didn't wear a tiara at her wedding?
Hi, Duchessmary. I hope this answers your question, but I have no official sources or interviews to corroborate my answers, as they are only speculation based on my observations of customs and traditions for the family and the time.
First, Princess Grace was not born a princess. Although she was a Hollywood actress and had played a princess before, she was not royal.
Second, at the time she married, most brides did not wear tiaras, only some royal brides wore them, as they were usually the ones who had them. And it was unlikely that she owned any of her own at the time.
Third, the former Grace Kelly was the daughter of a well-to-do but not necessarily wealthy American. Prior to Grace's notoriety as an actress, the family would not have been considered one of the wealthy, affluent, socially prominent American dynasties. Use of tiaras for weddings and/or other occasions was still only typical among the very wealthy like the Vanderbilts, the Rockefellers, etc.
Fourth, Grace's mother-in-law, Princess Charlotte, may or may not have liked her son's wife. Considering that in her will she left all of her jewels, including her tiaras to Princess Caroline indicates that she did not have a very good opinion of Grace (which is unusual considering that she herself was the bastard offspring of her father and a laundry servant). So it was also unlikely that she would have loaned a tiara to Grace.
Finally, even though some royal brides did wear tiaras, it was not the standard, even in Grace's generation. Many brides, including the royal ones, wore some sort of cap or wreath of flowers. This was true also for the Grimaldis, since Caroline also did not wear a tiara for her first wedding. Nowadays, it seems that many brides want to be a princess on their wedding day, so there are more of them, royal or not, who wear a tiara.
I hope that at least gives you some ideas as to why she didn't wear a tiara, but I suppose the only way to be sure would be to ask her. Tragically, that is not possible.
Have a great day!
Rascal