Because Medicare has, in a sense, too much market power and could unreasonably distort the market in favor of the short term incentive of getting the best possi

Because Medicare has, in a sense, too much market power and could unreasonably distort the market in favor of the short term incentive of getting the best possible price (the marginal cost of production) and against the long term goal of profitable drug companies that produce new drugs in the future. Now, there are plenty of people that don't find this reason compelling and thing that the US gov't should do otherwise. But that's the reason.