More than anything else, coal fueled the Industrial Revolution in the 18th century. It was, and still is, plentiful and cheap. It's also always been relatively easy to get at, especially if you don't mind sending kids into mines, endangering the lives of miners, or blasting the tops off mountains.
Among the various dodges deployed to point fingers away from the destruction that tar sands companies are inflicting on our climate is the argument that activists should in fact be fighting against coal instead. What those using this excuse don't know that there is a massive anti-coal campaign underway.