Because most people realize that everyone has different hairstyles for different events. You wouldn't expect a messy ponytail with a ballgown. I am a tomboy who lives in jeans and now scrubs, and a ponytail. But for prom, and now for formal work events, I can rock a proper hairdo when time calls for it. Doesn't mean people don't recognize you. You wouldn't expect a fancy updo for a day sitting in the grass at a casual polo match. Kate rocks beautiful updos when she chooses, at formal events.
The fact is a wedding calls for a hairstyle. When you are on display for millions, you need to be on point. And a hairdo that mathes a stunning tiara. Unfortunately Kate's hairdo did not do that. Fortunately she has become much better at tiara hair.
As much as we complain about Victoria's bun or ponytail, she rocks tiaras. She is one of those women who understand the concept of tiara hair.
Meghan is an actress. She has been on the red carpet plenty. She knows when an event calls to put on the full look and her wedding is one of them. Whatever tiara and dress she chooses, she will take the care to reflect the event she is at.
No one here suggests that we all should wear only one hairstyle all the time, because that would be clearly ridiculous.
But YES, for the wedding Meghan should choose a hairstyle that is her. It would be stupid choosing a hairstyle that maybe looks beautiful and it's great with tiara, but one she feels uncomfortable with/she doesn't feel like herself. It's her wedding day and she isn't there to compliment the dress and the tiara, she's there to marry the man she loves and wants to spend the rest of her life with. It's her and his day and not everything should be done just for the people who will be watching. All the things, the dresses and tiaras and flowers are there for her, not she is there for them.
My very good friend has a beard and had one for years (10, maybe 15 years), but for his wedding he listened to his mother and shaved it completely. And after some time he admitted, that he regrets that decision, because without the beard he didn't feel like himself. He even doesn't like his wedding pictures. Not to mention the shock the bride had, since she had never seen him without a beard
And yes, Meghan is an actress and she probably knows what to do to shine. And yes, Catherine clearly can rock a beautiful updo when she wants to. But red carpets and state dinners are something completely different. This will be Meghan and Harry's wedding and to me it's clearly more personal occasion than a state dinner/diplomatic reception/red carpet event. So she should feel comfortable with her hair, she should feel like herself to enjoy the most of her day. It shouldn't be required of her to put on a show to make people happy. Yes, the wedding will be (probably) televised, but it's still a very personal event.
As for Catherine's wedding hairstyle, it was clearly one she felt comfortable with. It doesn't matter that some people on the internet forum think she wasn't "royal" enough or it didn't went good with the tiara. She was happy. William was happy. They complied with the rules of a church wedding. Yay! for them.