Ultimately most of these decisions come down to economics and not mustache twirling villains. The banality of evil.

Simplest "bad" reasons are the wealth of the patients. Malaria, river blindness, guinea worm, etc are terrible diseases that mostly impact poor people out of sight from Western eyes. Spending $X billion developing a drug for a population that can barely afford to feed themselves is not going to make a financial return on investment.

Well, it would for those countries, but yes, it's not sensible for a private business with no guaranteed source of funding. If they did develop it and no one paid the PR would be even worse.