Because I want to see a tiara.
But seriously though, I think it'd stir up a lot of unnecessary controversy if she didn't wear a tiara. There has already been plenty of people doubting the Queen's approval at this union from the beginning. I'm not one of them, but I just don't think the BRF will want that type of narrative surrounding a high profile wedding. You add that to the fact that people always like to bring up Duchess of Windsor as Meghan is a divorced American. In my opinion, it's just bad publicity the royal family doesn't need.