Our model for developing drugs is stuck. Small molecules were unbelievably (literally) useful against invaders of our bodies. They can discriminate and interupt vital processes of one biology without effecting another's biology. This made the development of small molecule drugs in the 20th century against bacteria, fungi, and even some viruses verry effective. Cancer is you. There is no biologically 'other' one can use as a discriminant that doesn't apply equally somewhere else in you. Socially we've built an entire pharmaceutical industry whose talents are focused entirely on a methodology for finding 'interruptors' of foreign disease. And again, that is not cancer.
Look the the new CAR-T therapies for cures to cancer. They disrupt the pharmaceutical industry's methodologies in thier development. There can't be a tox test or a primate study. They are fundamentally customizable, cheap, and curative, but they are the injection of genetically modified immune cells containing invented and designed protein tools - not small molecules. There is literally not yet a vocabulary to distinguish these new therapies from the drugs of the 20th century. They are as distinct from drugs as vaccines - but have literally only been tested on a few hundred people, being approved by the FDA only in the past two months.