I bet it is because they want to operate mines also when it is dark. I'm interested if millettjon has any other explanations.
If that were true then solar panels really aren't that cheap. Having to run your mines inefficiently should be counted as a cost when comparing the two. Doesn't mean that solar panels isn't still the best thing, of course, just that his assertion that they're super cheap for the power companies is false.
They are starting to ramp up solar only in the last year or two. Endesa, the main hydro company, recently gave up fighting to construct 5 dams in the south due to persistent opposition over the last several years. Hopefully this will accelerate the solar deployments even more. The main mining company CODELCO is state owned. Endesa and CODELCO basically try to do whatever they feel like unless there is enough grass roots opposition. There is also a huge potential for wind generation in Chile which they are only starting to use.