A major reason for these shortages has been quality/manufacturing issues. However, there have been other reasons such as production delays at the manufacturer as well as delays companies have experienced receiving raw materials and components from suppliers. Discontinuations are another factor contributing to shortages. The FDA can't require a firm to keep making a drug it wants to discontinue. 

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With fewer firms making older sterile injectable drugs, there are a limited number of production lines that can manufacture these drugs. The raw material suppliers the firms use are also limited in the amount they can make due to capacity issues at their facilities.

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This small number of manufacturers and limited production capacity for older sterile injectables, combined with the long lead times and complexity of the manufacturing process for injectable drugs, results in these drugs being vulnerable to shortage. When one company has a problem or discontinues, it is difficult for the remaining firms to increase production quickly, resulting in shortages.