So, if we first focused on coal burning power plants, we could get a bigger bang for the buck as they pollute with normal pollutants, and coal is problematic to mine and the residual burned ash is also a problem. The advantage of coal, aside from low cost, is it provides steady baseline power, something you need if other sources are intermittent.
The obvious replacement is more costly, but pretty obvious, though the shift to natural gas has been obvious over the years, making more of an impression in our mix than wind and solar.