I personally think that, The dutchess of Cambridge is married and courted the heir to the throne. Papers sell because she will be our future queen as you could say, so people are interested in their life.
Meghan and Harry? Less so. They are not relevant. Their child is not relevant so no one really cares too much about reading every little detail, because they or their child will never be relevant in the royal family. So, to help the papers sell stories they make it edgy by either being defamatory or putting a spin on things for the shock factor.