Jonathan Chait cuts to the chase:

The main problem is that the Republican Party does not actually care very much about the deficit. It cares about, in order: Low taxes for high-income earners; reducing social spending, especially for the poor; protecting the defense budget; and low deficits. The Obama administration and many Democrats actually do care about the deficit and are willing to sacrifice their priorities in order to achieve it, a desire that was on full display during the health care reform debate. Republicans care about deficit reduction only to the extent that it can be undertaken without impeding upon other, higher priorities. Primarily “deficit reduction” is a framing device for their opposition to social spending, as opposed to a genuine belief that revenue and outlays ought to bear some relationship to each other.