A number of CBO-watchers were asking that question when the agency started scoring health care bills a few months back. As Washington and Lee law professor Timothy Stoltzfus Jost put it in Politico:

It is much easier to score costs than cost-savings. Legislation pending in both the House and Senate in fact includes state-of-the art proposals that many health policy experts do believe will result in real savings, as the CBO recognizes. It is easy, however, to figure out how many people are under a particular multiple of the poverty level and how much it will cost to cover them through Medicaid or to provide them with insurance subsidies, i.e the cost of reform. It is much harder to figure out how much public plan choice or accountable care organizatons will save the federal government. The CBO guesses conservatively with respect to savings, and the media reports this as a "blow to reform."

Now, in an interview with The Washington Post's Ezra Klein, CBO director Doug Elmendorf responds to the criticism: