The reality is, the government needs money to invest in hospitals and schools and roads and defence and such forth. It can get the money from income tax, but that's generally unpopular. Taxes on big shopping stores like Safeway and Coles are also unpopular, because those sorts of taxes are always directly passed onto consumers. If they tax the mining industry, consumers may pay more for certain goods and services, but it won't really directly hit them the way other sorts of taxation would. (This is debatable, I know that the mining industry doesn't argue this). The idea though, is that with the money gained through the mining tax, the government could invest in infrastructure so that there is more diversification in Australia's industries. Which means that when the mining boom inevitable crashes, we still have other investments which could keep us afloat.