I think you hit the nail on the head. I think they have not earned it yet.
If you think of previous generations: Q Mary & K George, also, K George VI & his Elizabeth were taking on full loads practically from the honey moon. These two have made it known that they do not yet feel quite ready to go full time.
The Cambridges & P Henry are now adults in their thirties. It is time they got on with it more seriously than attending a few sports matches. Their ageing relatives need to be allowed to step back if they want to.
I know I am likely to get a lot of grief for this post, but it is how I feel.
Ana