That's not obvious at all. People in coal-producing areas voted for Trump for the same reasons that people in wheat-producing areas did: they're rural Republicans who watch a lot of TV and the Democrat candidate never even visited their congressional district. Most of the people in those areas don't actually mine coal, and they knew they would get poorer on average under Trump just as they had gotten poorer on average under Obama, Bush, and Clinton before him. They just appreciated a moment of lip service. We can't vote in USA without pretending at some level that voting matters, so why wouldn't they also pretend that one of the candidates cared about their interests?