Quite puzzling (seen from abroad) that the Dutch King is not allowed to wear an uniform for such a gala occasion.
The King could wear a uniform which would be considered as a civilian uniform rather than as a military one - just like ambassadors or high-ranking courtiers can wear court uniforms. The morning suit seems strange and disappointing - especially when compared with the Queen's grand gala attire, with the Golden Carriage, with the coach's drivers and footmen in full-gala livrée...
I am sure the Dutch democracy would not be harmed if King Willem Alexander was dressed like his great-great-grandfather King Willem III: