Why is so much less money spent on diseases (prevention)that are so much more common (e.g., cancer)?
There are two sources of funding for medical research, private funding and public, i.e., government funding. Government funding must be distributed to a panoply of research needs, and it must be able, to some degree, to justify its choices in the use of taxpayer dollars. Private money, from universities, from foundations, from contributions, and from businesses such as pharmaceuticals, does not require the same kind of expenditure justifications. Without knowing exactly, I would hazard a guess that there is more private funding for medical research than there is government funding, and I would also guess that most of this is driven by pharmaceuticals. Pharmaceuticals fund research that they hope will result in profits, and this usually means cures and treatment for symptoms, not preventions. If people already have a disease and the research yields a medication that cures or alleviates symptoms, the pharmaceutical will make money. Prevention of disease is a worthy goal, but not a money-maker. Additionally, we already know how to prevent, or at least, reduce the risk, of many diseases, for example, some forms of diabetes, at least a few kinds of cancer, and sometimes hypertension. There is no profit, certainly for pharmaceuticals, in encouraging people to make lifestyle changes.