According to a doctor I know, one of the big factors is (as we all expected) regulation. In particular, regulations have increasingly mandated shorter expiration dates on drugs. According to this doctor, this sort of regulation results in tons of perfectly good drugs and precursor compounds being discarded because they have reached an arbitrary date printed on the container. Many of these are known to be chemically stable for years or even decades under proper storage. Another factor he cites is the increasing rarity of compounding pharmacists. For various reasons, mostly regulatory and liability, institutional pharmacies (i.e. at the hospital) are more likely to depend on off-the-shelf formulations whereas in years past they frequently compounded formulas out of readily obtainable precursor chemicals. He said some of the drugs on the shortage list are not actually in short supply from a chemical perspective, but in short supply in certain forms, such as injectable solution. One example he listed (which I now forget) is something sold in capsule form at health food stores everywhere, but in its IV form it is in short supply. He says a pharmacist worth his salt could make up an IV solution in-house from these readily available capsules, and that "in the old days" that was one of the fundamental skills of a hospital's pharmacy staff.
