Yes bluetoria, 4 what I know is that prince Michael the only was to attend of all the Royal Houses.
Maybe there was a sort of protocol. Its custom that if you get an invitation of the host country you as King/Queen or one of the childeren go, to a burial/funeral, wedding, etc.
I think that Poetin didn't send any invitations to the Kingdoms of the Netherlands, Belgium, Norway, Sweden, Denmark, Spain, etch etc.