The rich pay most of the taxes. When their incomes fall, the government revenue takes a hit. Remember that taxation is basically a form of theft, and as any smart thief will tell you, it makes a lot more sense to rob from the rich than from the poor.
Politically, the problem is that the Democrats consider anyone not living “hand to mouth” to be rich, whereas the Republicans perceive “rich” as someone who really doesn’t have to worry about money. So to the Democrats, most of the middle class is “rich” and fit for heavy taxation.