Cancer is about two hundred diseases, at least. It's a case of undesirable evolution: cells evolve toward radical individual fitness at the expense of the organism. The body has ways of protecting against that, but the most "fit" cancer cells can sometimes overcome those. It's an ongoing arms race. This selection phenomenon (clonal evolution) similar to anti-biotic resistance is what makes it so hard to treat.
This is one reason why I have little patience for creationists. One of the most important problems (curing cancer) is an issue directly involving evolution (at the cellular level).
The cancer research is returning a lot of yield, but this is just a very hard problem. The body has about 10 trillion cells. Some are good, some are bad. The bad ones become increasingly able to survive treatments due to clonal evolution. The good ones die as well, because these are systemic treatments (highly powerful drugs) that interfere with cell reproduction. You have to kill all the bad ones, or the cancer will probably return. If you kill too many good ones, the patient dies a horrible death that's much worse than a regular cancer death. It's not easy to do.
It's just a fucking hard problem. We should be putting 10-20 times as much money into it. How many things are more important than fighting cancer? Not these fucking social media startups.