Maybe... to mark her first birthday?
If they don't share pictures they're accused of being secretive. If they do share pictures they're accused of trying to steal thunder.
You seem to question why they didn't show her full face in one photo. You also seem to question why they didn't show a photo of her closer to her birth.
And now you are questioning why they released the photo on her first birthday which is a very common day for people (including public figures) to share a picture. Including Kate and William's children who they share an official photo of every birthday.
Meghan and Harry can't win in the eyes of some people, whatever they do, it's baffling,
I think that the Queen would be satisfied that she got plenty of coverage for the Jubilee and it wasn't overshadowed by a picture of a baby so no need to worry about that.