First of all, the basic science is lacking. The vast majority of animal models in no way resemble the normal process by which humans develop cancer. The models are designed to grow aggressive cancers extremely fast so you can test drugs rapidly, publish your paper and get the next grant. Even more than that, what actually takes place in the years before a cancer becomes clinically relevant is quite speculative. So a long term commitment to funding prevention research is needed from governments and funding bodies, because pharma can't fund this kind of basic science.
The other problem is that to do a good prevention trial, you need to identify people at risk. We aren't really that good at doing this (eg we screen ALL women above the age of 50 for breast cancer, we screen EVERYONE above the age of 50 for bowel cancer). There is some debate about whether screening in this manner is actually as worth wile as it has been made out to be, because it leads to over diagnosis without a survival benefit. So you have to be careful not to design an intervention that only prevents cancer in those who weren't going to be affected by it anyway. This is why you can't use surrogate end points - at the end of the day, for prevention, mortality is what matters. Are we going to give hundreds of millions of women drug X and make them suffer niggling side effects for 20 years because it reduces the rate of diagnosis of breast cancer? Definitely not - overall mortality is what matters (and quality of life).
It's not even a given that you can actually identify people at risk - some researchers think cancer is just bad luck (ie it is not predictable at all, or the predicting factors are unknowable). Even if this is wrong however, cancer risk probably involves weak effects from hundreds or even thousands of factors. We are very far from working all of this out.
A drug is probably not what we want. Actually a drug is really not what we want. They are too expensive and too much effort to distribute and monitor. Drugs also always produce health inequalities within and between nations. A holistic approach to improving our health and well being would be preferable, at least incorporating nutrition, exercise and psychology. Unfortunately the nutritional, exercise and psychological sciences are not hitting home runs in that regard, which is not surprising because they miss out on the billions we spend fighting an incredibly wasteful war on cancer.