TBH I think it's partly a snooty reaction to the upstart American, but also that there's a strong unacknowledged undercurrent of racism in all of the negative press. Like, if this divorced American actress is going to come over here and steal our ginger prince the least she could do is be white!
Personally, I like Meghan and I hope she continues to just be fabulous in the face of the criticism. It'll die down eventually. People generally like Camilla now, I believe, but fifteen years ago she was painted as a scarlet homewrecker. The UK press is full of garbage mudslingers.