Money? These drugs and the companies don't come cheap.
Take Gilead for example, which created Harvoni/Sovaldi for treating Hep-C. It's currently valued at about $150B. Before the release, it was a quarter of the price, so the value of Gilead's Hep-C franchise is in the order of $70-$80B at the very least. That's about the entire net worth of Bill Gates.