It was politics from a couple decades ago. Medicare historically didn't cover prescription drugs, until Part D was enacted during the Clinton administration.

A tactic to get Republicans to support the measure was to pledge that Medicare wouldn't interfere in the drug market by developing its own formulary – Medicare Part D plan providers would compete for members and negotiate with pharmaceutical companies themselves.

There has been a bunch of talk over the last decade about repealing that particular restriction, but it's never made it over the finish line.