if you're a person who mostly cares about profits which one do you accept
research 1: we're on our way to cure [rare illness]! we just need funding to find and develop the proper formula, we need x million for the budget, and the drug itself is not likely to be expensive and will be one time use/short therapy
research 2: we're going to tackle [not really common but not quite rare illness], we need x million for the budget, we probably won't cure it but a weekly dose of the drug will help those affected
now think like the only thing that matters is your profits, research 1 will cure people, and sure you could make the cheap drug cost $100000 but the researches could turn against you and release their research to the public losing you profit. and even if they don't you'll need to balance the price to turn in some profit in as short amount of time as possible. if the illness is rare that means there isn't many people who are affected, and those people are not likely to be rich - why bother
research 2 on the other hand is an easy investment, people will need that drug forever so you can set the price low enough for most to be able to barely afford and get your sweet sweet money back with profits fast
remember you don't become a billionaire being charitable, you become a billionaire by cutting corners and milking as much money out of those below you as possible
This ignores the very nature of pharmaceutical research and development.
Pharmaceutical companies aren't really research institutes, because research and development is terribly expensive. The primary research of just about any major drug innovation is typically first pioneered by Universities who are publicly funded.
A Pharmaceutical company's version of research and development is taking the primary research done by universities and developing them into a drug that is patent protected.
There is a ton of rat fucking in pharmaceutical companies that lead people to this type of conspiratorial thought, but most of it is pertaining to patent law, not dictating what a bunch of grad students are doing their research over.
Most chronic illnesses are the result of accumulated damage and dysfunction that are diffuse throughout the body. Something like MS has done damage to millions of nerves by the time it gets diagnosed, and the body is not particularly good at healing nerve damage to begin with.
Chronic illnesses almost always require chronic treatment because of the nature and extent of the damage.
Quite the contrary, there are some rare illnesses that are unique in that they do have actual cures now. For example, the Sickle Cell gene therapy is the first gene-editing treatment to be approved for human use and completely cures the individual patient of Sickle Cell disease.