Over the last thirty-five years, employment in the coal industry has declined.
The decline is the result of various factors: falling prices for natural gas; advances made in renewable energy technology-- particularly in wind and solar energy; federal regulations to protect public health caused additional but necessary cost-- which Trump is determined to roll back; mechanization, automation, and robotics have caused unemployment.
All those factors have created falling demand for coal and decreases in employment.
Robert Reich puts it in perspective in this Inequality Media presentation:
What’s Driving the Decline of Coal in the United States