Two main reasons I think.
1) There is a general wish for such photos to be released on the children's birthdays. - Just look at the interest and expectations here.
The tribe or public if you will, follow the family continuously. It's good PR and probably the parents wish to have photos taken anyway, so why not publish a few?
I think the public, and the press, would find it very odd if Joachim's children were no longer photographed on their birthdays. It would IMO backfire seriously and there would be endless speculations as to a strife within the family.
2) In order not to treat the children differently. I think it's pretty sure Joachim's children will all end up having to support themselves, with few royal duties. - Unless they wish to and have a talent for public duty. But until that has been decided, and that may take many years, it's simpler to treat all the children the same.
However, I think the last official birthday set to be featuring Nikolai will end when he turns 18, and then only at round birthdays and special occasions or for family portraits.
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