No information about the current ownership of the big diamond tiara was leaked. It is not publicly known who the current owner is now.
My guess is that Pavlos has inherited it from his grandmother, and that some obscure reasons prevented it from being worn for so long. But that is mere speculation, and other posters have other theories about it.
About the fringe tiara: Maria Olimpia is now 18! Do you think that she'll wear the fringe at the next best event in Copenhagen? Maybe the Queen's 75th birthday next year?