That is because the Cystic Fibrosis foundation funded some of the research and drug development to make the first treatment possible. They had to essentially operate in a VC model to get the treatments that these CF patients need.
Most disease categories have advocacy and funding groups, with varying amounts of success. The point is that rare diseases get new treatments all the time from private for-profit firms. In fact, orphan diseases are extremely profitable, and the FDA offers fast tracked approval decisions, making them an enticing niche to focus on.