Cancer is an individual cell reverting to what cells have done for 4 billion years. Multicellular organisms have evolved ways of preventing individual cells going rogue.
It takes eight to ten mutations in the same cell for cancer to begin. Oncogenes get stuck in the on position and tumour suppressor genes are locked off. It's a bit like a car developing faults with the accelerator, brake, steering, clutch and ignition switch one after the other so that there is no way to regain control.