* Most drug candidates just don't work
* Even among the drug candidates that do, figuring how to safely deliver them to their target is very hard (looks similar to "just doesn't work")
Bad reasons:
* It's too expensive to prove that a drug works
* It's too difficult to differentiate the patients for whom a drug works and the patients for whom it does not
* It is very hard to predict recruitment and to actually recruit patients for clinical trials
* There aren't enough people with the disorder who are also rich enough to afford treatment to justify development