The uniqueness of this event is that the Atacama Desert is a 600-mile-long plateau known to be one of, if not the driest and most sterile deserts on Earth. Because moisture is blocked from the east by the by the Andes mountains and from the west by the Chilean Coast Range, average rainfall is just 0.04 per year and skies are almost always cloud free.
Some weather stations in the Atacama have never received rain, and research suggests that some identifiable river beds have been dry for 120,000 years.
The region is so unique that NASA uses it to test instruments for Mars missions. Tests of the instrumentation of the Viking I and II Mars Landers for detecting signs of life, for example, found none in Atacama Desert soil. Also, because of its unworldly appearance, the Atacama has been the set for several movies and TV programs (e.g., Space Odyssey: Voyage to the Planets)
See also: Good video from BBC.
The recent snow occurred when an extreme Antarctic cold front, the strongest in at least 30 years, managed to break through the otherwise almost impenetrable rain/snow shadow. (Note: give the elevation of Atacama, about 10,000 feet, and extreme cold, the water content of the snowfall was likely at least 50 percent above the nominal 10:1 snow/rain rule of thumb).
Further south in Chile up to eight feet of snow was reported leaving thousands of people isolated without electricity and supplies (here). According to one regional governor, “In four days we have had four months worth of snowfall.”
Whether related to global climate change or not, chalk this up to one more on the long list this year as an extreme event.