I do think they were invited, but declined for various reasons. My best guesses:
Dutch: WA and Maxima would have been there if it hadn't been for the fact that they are king and queen now. The swedish and dutch royal families are close. The other members could have had previous engagements.
Spanish: Problems at home. Not good PR to go to a wedding with lots of bling and fancy stuff in a small, obscure nordic country if you have domestic issues.
Belgians: No idea, actually...
Belgians --> CP Philippe is in USA, but another members like Princess Astrid with her husband? - no idea
Spanish --> what problems? economical? Inaki?