They're fair questions.
1, Realpolitk) Because our democracy gives states power outsized to their population through the Senate, and because there are several purple states along the coal belt.
Consequently, their voters (many of whom are miners or employed because of downstream mine revenue) possess to the political power to force people to listen to them.
2, Moral) In 2014, per capita income for the region was $37k, or about 80% of US. Poverty was 19.7% vs 15.6% for the US [1]. From personal experience, there are still places that feel like 1950. (Also, per capita, the area has been and is disproportionately over-represented in our armed forces)
3, Economically Practical) As others noted, there isn't exactly a humming engine of economic diversity in the region. Heavy industry and mining is vastly overrepresented, which generally means if "the plant" closes the town dies.
4, Cultural) From music to speech to food, there's a distinct culture there. So moving "elsewhere" isn't just a geographic change, but more akin to uprooting thousands of people from New Orleans and dropping them in other cities.
As another commenter suggested, "Why not just move people away?"
Honestly, I'd be fine with this, if it were done humanely.
But screwing people isn't going to win votes (or feel particularly morally righteous). And short of that, we aren't going to get the environmental change we need.
Hell, we could have probably transitioned the entire region off coal for a fraction of the budget we spent on the Iraq War.
[1] https://fahe.org/appalachian-poverty/