Because coal creates a ton of pollution. I'm sure he doesn't want to encourage Global Warming. Al Gore certainly wouldn't endorse him.
Global Warming vs Strong Economy
Right now China, India and Australia are mostly powered by coal... China and India owns have the majority of the population. So you tell me...We are exporting coal like crazy and telling people not to use them. So much contradiction all politics...201
1 saw high levels of coal export
http://www.sourcewatch.org/index.php?title=U.S._coal_exportsUS coal exports increased rapidly in 2011, returning to levels not seen since the early 1990s,[1] and accelerating to keep up with rapidly rising global demand.
The U.S. is the world's forth-largest source of coal exports in the world, after Australia, Indonesia and Russia (1st, 2nd and 3rd, respectively). Unused port capacity on the east coast and major expansions proposed for the west coast mean that U.S. coal exports could rapidly overtake Indonesia and Russia.
We also have more coal than anyone on this planet with 25% of the world's coal. So does it matter if we use coal or not? It doesn't.
“Because the impacts of CO2 emissions are global in nature, it makes no difference from a climate change perspective whether coal mined in Wyoming is consumed in Chicago or Shanghai,” said a July, 2011 report by the Columbia Law School’s Center for Climate Change. “With coal export volumes poised to increase dramatically in the near- to medium-term, circumstances call for more comprehensive legal and policy response.”
The reason for coal to drop can be due to new regulation like clean air act...which is good but doesn't matter according to the above quote.