Because it's less expensive. And it isn't exposed to forward price volatility of fuel.
This is more important to most companies than cheapest price. It enables them to plan with more certainty.
For coal we should look at all the health costs cause by mining and burning coal.
For oil we should not only look at the tax breaks the companies get but also how much of the military (percentage wise) exists to protect oil interests.
The health costs of the renewables are a lot less.
Although, the USA is about 90% energy independent
https://en.wikipedia.org/wiki/United_States_energy_independe...