Because drugs that only help a handful of people are excellent bargaining chips for pharmaceutical companies when dealing with national governments for coverage

Because drugs that only help a handful of people are excellent bargaining chips for pharmaceutical companies when dealing with national governments for coverage of drugs in national healthcare insurance plans. It's the basic economic law of supply and demand applied to human lives. The same happens (or happened) with the drug that treats Pompe disease, which is currently priced at a sweet spot high enough that enough insurers can cover it to make massive profits (but some refuse to, because the amount of money spent on it can be used to save more lives when invested in other areas).