The answer is, of course, that curing a single cancer, much less many cancers, is really, really hard. Scientists not only have to contend with basically a continuous spectrum of mutations between individual cancers that renders the concept of even single cancer subtypes horribly naive, but with a near continuous spectrum of mutations between groups of cells in a single tumor.
There is reason for hope, however. As was discussed at AACR 2012, the vast majority of cancer-causing driver mutations can be divided into 12 key molecular signaling pathways, and mutations at the “trunk” of the divergent branching evolution tend to remain as divergent evolution occurs, and targeting them is likely to be more effective than targeting mutations further out on the branches. It might be possible to hone in on these different pathways. In the meantime, as the cost of sequencing continues to fall, it might become feasible to sequence several parts of a tumor and its metastases and map out treatments that cover all of them. Personalized medicine is thus possible and still holds promise. It’s just not going to be as simple as getting a biopsy of a tumor and picking a targeted agent or two to blast the tumor into oblivion. The point is not that personalized medicine is impossible or that it’s impossible to develop cures for various cancers. The point is that biology is always way more complicated than we ever thought it was, and evolution almost always wins out. Even so, the reason we haven’t cured cancer yet is because we haven’t figured out how to overcome the power of evolution. Until we figure out a way to do that, we will continue to make only incremental progress.