Imagine that grand jewels were obligatory: many, many ladies at this event simply do not have a diadem. We even do not know if the Princess of Monaco has a diadem or if she is in the mood to wear that particular jewel.
Wearing Orders is also an option and never obligatory. There are enouugh people whom were honoured with a high Order but feel uncomfortable wearing such an item. For an example, Mr Behn is a Commander in the Order of the House of Orange. He has a neck badge (like Mr O'Neill today) but decided not to wear his (only) royal Order.
And again Queen Fabiola comes handy: she never wears Orders except a Vatican one. Her own choice and she is perfectly entitled to.