The value rarely is in the jewelry, but rather in the jewels themselves. So, if you had stolen, for example, a gold ring with a big diamond, you’d take out the diamond and either sell it as-is, or chop it into smaller diamonds and sell them, and probably have the gold smelted back into raw material again. From what I know, it apparently is common for gems (especially very valuable ones) to be given as gifts or traded and fitted into new settings, like rings, necklaces, crowns etc. over the course of centuries, not just stay in the same piece forever.

What I think is most likely in this case, is that some rich asshole wanted to have them for their private collection and hired someone to steal them.