I was expecting this. She never, with the exception of Peter Philips (and that photo was not taken at his christening), was photographed holding one of her grandchildren.
The Queen Mother was pictured holding both William, Harry and Charles.
Even dignified Queen Mary was photographed holding Charles and Anne at their christening, not to mention Queen Victoria at the famous 4-generation pic....
Well, the current Queen has, for whatever reasons, her strict principles (the same goes with her refusal to attend important foreign royal events) which are even for me, a staunch supporter of HM since my childhood, hard to understand! Little gestures without any great efforts could make a huge symbolic impact.
I also think it would have not been too big of a strain for the Cambridges to appear for 2 minuets in front of the palace doors after the christening to show some respect to the many faithful admires who waited for so many hours, some even with gifts!
Many regard this (to make a little impact as possible) as a "modernisation of the monarchy" - I fail to see what´s "modern" about this...