They are Britain's richest noble family by far but even they have sold off significant jewels in the past two decades. There are pictures of the now Dowager Duchess wearing huge jewels (the Bagration necklace, the Rosebery tiara) in the 1980s, however she then either stopped attending those kinds of events or they dried up.

I suppose a lot of these family's will always keep a tiara or two to wear at weddings but they don't need a huge collection of jewels, and even if the families were hugely wealthy like the Westminsters, they may be very conscious that if they can get £500k for a tiara they would rather invest that money, as opposed to keeping it in a vault for 20 years at a time.