Mining knowledge was not written down.

If the people who held it left, the knowledge left with them.

A third possibility: simple resource depletion.

The most accessible mineral deposits in the cave had been worked for centuries.

Getting to fresh, unworked deposits required going ever farther into the cave’s upper passages — reaching the limits of what cane-torch navigation could support, or reaching passages where the mineral concentration was too thin to justify the journey.

The operation may have become economically unviable in a practical sense as the most productive areas were exhausted.