For the Danish and Norwegian royals I would say its probably just a case that no one was available.
In Denmark Fredrik and Joachim are away on holidays with families so that only leaves the Queen and Princess Benedikte. I suspect as Anne-Marie was there she may have represented both her husband and sister.
In Norway the King has recently been ill so an event sitting outside in December may not have been the best.
I think it also depends on the personal relationship.
From Monaco only Prince Albert has personally known the former King. The monégasque princesses usually play no role in foreign representation.
From Denmark Queen Margrethe and Prince Henrik have personally known the late King. The Prince has retired. The four other members of the Royal House are from a far younger generation and have probably never met the former King.
From Norway King Harald and Queen Sonja have personally known the late King. The King is in frail health. The two other members of the Royal House are from a far younger generation and have probably never met the former King.
From the Netherlands Princess Beatrix has personally known the former King. The only two fulltime members of the Royal House (King Willem-Alexander and Queen Máxima) are from a far younger generation and have probably never met the former King.
When Monaco, Denmark, Norway and the Netherlands are represented by their Ambassadors, they are "represented" of course. Anyway, we may assume that letters will have been sent, as well floral tributes.
King Juan Carlos and Queen Sofía have personally known the former King.
The same for Archduke Lorenz and Archduchess Astrid (Lorenz was one of the King's counsels).
King Carl XVI Gustaf and Queen Silvia have personally known the former King.
Grand Duke Henri has personally known the former King.