That’s a good point. Diana came from a titled family; her father was a Lord and 8th Earl Spencer. Kate was a Commoner. Possible class issue at play? Also, Kate & William have been at the forefront of “normalizing” the Royal Family, showing a more informal style. Perhaps that is a factor, too.
I dont think its a class issue. The class system doesnt really work like that. I think its more that she was Kate Middleton and that name has kind of stuck. I know she doesnt like being called Kate anyway, preferring Katherine.