From the article, seems like it was due to unique geological properties.

Geologically speaking, the uniquely pure minerals at Spruce Mine were created about 380 million years ago when Africa collided with North America. This momentous collision, however slow, caused intense friction and heat miles below the Earth’s surface. According to Sibelco, the Spruce Mine minerals were created by a rich mineral-forming liquid that cooled and crystallized over time. A standout feature of these minerals is that they were made in their purest forms due to a lack of water, which caused all the friction.

It does later say that similarly high purity can be reached with synthetic quartz but that it would take time to scale up that industry.