Because of the way drug patents work:
The duration of patent protection in the U.S. is 20 years. All drug innovations get patented at the time of discovery, but late-stage cancer treatments will work their way through the clinical trials required for FDA approval much more quickly, since the effect on patient survival will be apparent within a couple of years. That means fewer years of the patent clock ticking without revenues coming in. For a preventive treatment like the HPV vaccine, the 20 years of patent protection will be long expired before any clinical trial can show whether lives are being saved, which in turn makes vaccines far less alluring investments for biotech companies. It’s yet another indication of America’s patent system’s desperate need for a makeover…
Generally, to get approval for a new cancer drug you need to show that patients live longer. It doesn’t take long to determine whether a new treatment adds months of life in the case of metastatic cancers (those that have spread throughout the body): 90 percent of patients with such cancers are dead in less than five years. But it can take more than a decade to see whether survival is affected for localized cancers that remain confined to a single organ. And for treatments aimed at cancer prevention — the holy grail of cancer research — it could take multiple decades to prove a treatment’s efficacy.
Full Ray Fisman post worth reading.