How come?

Much has been made of the closure of the country's last deep coal mine.  End of an era, and all that. But what struck me is why it is closing.  Not because it's run out of coal - quite the reverse. There are still millions of tons of the stuff down there.  No, it's closing simply because it's cheaper to buy coal from abroad. So we're still going to be using coal, but somehow it appears it's more expensive to dig it up in our own back yard, than it is to transport it thousands of miles from places like Russia or the US. And I think I am entitled to ask just why that is so.