Because the mining industry is powerful (major share of our exports) and big business dominates the media. They get the public fearful about the loss of jobs.
Further, Labor's infighting put off a lot of people while the Coalition were consistent with their message in opposition.
Mining companies own, or control much of the Australian media. There was only one newspaper who didn't go for the coalition in the last election. It was the Age, which was mostly critical of Labor, but not entirely. There were big legal battles for control of the Age, but the existing board one control in the end, but lost a lot of editorial power.
Out an out corruption basically. The country began because of a miners revolution, and to this day they have more power than they earn.
The large coal mine (by an Indian company) despite declining coal prices, and the state earning more money via tourism, is going ahead because of brilliant control by the mining companies. This company has a terrible record of keeping their mines clean as well. Not only that, but the carbon produced is similar to all planned carbon saving planned by the government proving they lied about reducing carbon themselves. Further more coal kills a million people per year through pollution, and China is reducing their spend on coal, like most other countries. Which means the price of coal is only going to fall.
Further, Labor's infighting put off a lot of people while the Coalition were consistent with their message in opposition.