For what reasons might nations of the world have agreed not to begin mining operations in Antarctica

In evaluating risk, there are some issues where the risk of miscalculation would create such a catastrophe that it isn't worth taking that risk even if the evidence says the risk is very low.

Antarctica is a pristine environment that, like the Arctic, is critical to the survival of the Earth. There are materials there that the greedy can make money mining but there are no materials there worth jeopardizing the planet to get them. Sooner or later, we humans have to learn that the Earth's resources are limited. Personally, I'm in favor of sooner.