There is nothing to explain because I made no such claim. I’m not sure how you could arrive at an assumption like that. Did you read my comment or did you see the phrase “big pharma” and fill in the rest with assumptions?
Pharma companies make money on patentable drugs that they can monopolize. They pursue the avenues that are likely to be most profitable, and profitability increases when there is less competition. A 100 year old drug that cannot be monopolized is a less profitable pursuit than others which can be monopolized. The condition to be treated is beside the point in the math of profitability.