Pharmacy by Army Medicine (cc by)
There are many reasons, but the primary one is that the massive series of mergers and buyouts in the pharmaceutical industry over the past ten years has meant that companies have consolidated their manufacturing facilities. Where in the past you might have had six or seven companies manufacturing a certain drug in eight or ten factories around the country, you now have one or two companies making it at two or three sites. If one of those sites goes down do to equipment problems or regulatory shutdown for improper safety standards (that's what happened to Novartis -- they got hydrocodone in their Excedrin bottles, allegedly), that means the other sites (if any) can't keep up with the increased demand.
RockSteady explains why we're seeing more and more prescription drug shortages lately, and why it's a big problem.