I've answered this nonsense twice already on this forum. Basically, when experience suggests you can reliably find stuff by going to look for it, you don't bother going to look for it until you need to.

An awful lot of these rarer elements come out as byproducts of mining other things, eg zinc, etc, but you don't bother extracting it unless the price is high enough. The prices of most of these things fell to low levels a few years ago, and some mines stopped bothering extracting them. The prices have mostly gone up again now, a lot, and they'll start extracting them again...