Meghan is mixed race and doesn't have the strong coils of most African American women. Unfortunately (and incorrectly), afro and very curly hair is seen as bad, unprofessional and many other negative things.
It is also hard to maintain (it needs to be moisturised every day and braided or covered up at night, gets tangled easily, etc). Relaxing this type of hair saves time and makes it easier to work with. It is Meghan's choice what she does with her hair, some people would embrace her natural hair and others would criticise her for it or see it as 'not as good as' smooth hair.
Most famous women have a lot of work done to their hair - always styled, regularly dyed, straightened, curled, wigs, hair pieces, sponges, etc.
Kate is said to have wavy hair naturally, but it is straightened or straightened and then curled. William has said that her hair is 'a nightmare' and that it takes a long time to do. Additionally, 60% of Europeans have naturally curly hair (I'm studying trichology), but most women seem to straighten it or straighten and then style. In the past, many women curled their hair.
Most women seem to dye their hair too, even if they have little to no grey. Both Meghan and Kate have sported many different hair shades (and why shouldn't they?).
There is a lot of pressure on black women to have 'good'/'acceptable' hair (more so than other women), both from other black people and non black people. There is definitely pressure in a professional setting.
If anyone wants more information, Emma Dabiri is a good writer and Chris Rock's documentary 'Good Hair' is interesting too.