Because diamonds are a shitcoin that can be easily synthesized, whereas the smartest brains of humanity have spent millennia getting destroyed trying to figure out how to synthesize gold. Perhaps more importantly, diamonds can be destroyed, whereas gold is indestructible. This means diamonds' liquid stockpiles are constantly being consumed, and new production is therefore significant compared to existing stockpiles (low stock-to-flow ratio), whereas gold's indestructibility means new production is always a small fraction (1-2%) of existing stockpiles, which are the accumulated sum of thousands of years of production. So even if you find a way to drastically increase the production of gold, you will add more supply to the existing stockpiles, which means the annual supply growth rate will remain low. As the new method is more widely adopted, the stockpiles keep rising and supply growth rate remains low. The best way to kill gold is to do what governments have done in the twentieth century. Set up a government monopoly on banking, collect a lot of the gold, give the plebs paper receipts redeemable in gold, stop redeeming them in gold, and ban the plebs from building their own banking system.