It's not just rare earths... lots of minerals (most glamorously, gold) are this way. There are mines that are profitable to operate if the mineral is selling for $x per kilogram (or gram, or tonne), but are not profitable to operate if it's selling for $x-1 per unit (or any other figure less than x). Shutting mines down and (sometimes) starting them back up based on economic reasons has been going on for centuries.