@CurlewKate thanks for your explanation. I agree with it. Whilst it seems mad to see a successful actress and a senior member of the royal family as underdogs that is how they have clearly positioned themselves over the years since they found freedom. Put that together with the fact that in the US most don't understand how the monarchy works and how it intersects with our culture and politics then i can see how they pulled it off.
I think we also need to remember that for the US issues of race are very different than they are here. We have never had legal restrictions on what black people could and couldn't do; in the US segregation didn't end until 1964 and that wasn't the end of restrictions just the beginning of the end, it is all very recent. If you spend any time in the US a lot of areas still feel fairly segregated in terms of housing and other things and since Trump things have got worse not better.
I'm not saying the UK is perfect, we can be very racist it just isn't a simple as black/ white (remember the no blacks, no Irish signs).
So to pivot back to the Sussex underdogs. They positioned themselves as cast out from the evil royal family, they hinted at racism and made sure to suggest that the UK Commonwealth is slavery 2.0. I can see why people who have been or are subject to racism in the US might see Harry and Meghan as people who understand their own struggle and worthy of support.
As an aside I love protocol and tradition. It's a thousand year old institution, I don't want the top job to change hands with a hand shake or a signature. I want ancient spoons, secret oil recipes, posh jewelled hats and rules about who walks where and curtsey to whom. It makes it fun and weird. And the world needs more fun and weird. I'd hate to be part of it but I like watching.