They use Atlas V because they already have the engines in inventory. They are already paid for, manufactured and waiting to launch. All the remaining Atlas V rockets launches were also scheduled long ago and the payloads were designed to fit in its fairing.
Because Antarctica is nothing like the moon. No water, no snow or surface ice, no wind, no penguins on the moon.
No dangerous radiation, no dusty regolith, no vacuum, no baking temperatures in the sun, no microgravity in Antarctica