I take it as choice of the parents. While Victoria & Madeleine tend to want more privacy for their families, they understand their positions and interest in their children so they have more control on what's released to the public.
Prince Oscar's last photo op was when he was presented to Sweden (and the world) on the king's birthday (the balcony), in the arms of his mother, the heiress apparent. The balcony presentation showed a grumpy but cute prince (I want my nap) but it defined his station. And Oscar's christening at the Royal Chapel where Estelle was christened also reminds everyone he's the child of a future sovereign.
Alexander looks adorable in the new pics.
I agree about the christening, but I don't think Oscar's appearance on the balcony has any meaningful significance in terms of his status within the dynasty. Putting it in other words, I am pretty sure that, if Alexander were old enough, and had not been born only a few days earlier, the King would also have him in Sofia's arms on the balcony. Future events will confirm if I am right or not.