I think it's because people grossly underestimate the impact that diet and lifestyle have on health - they remain the most important causes of the biggest killers in Britain and, in many cases, the most important treatments. Taking a pill has never been, and probably never will be, as effective as a long, hard look at diet and lifestyle and taking sensible steps to modify them.
Heart disease is a good example.
Despite major advances in the treatment of heart attack, the single most important intervention, in terms of prolonging long-term survival, remains to persuade patients who smoke to quit. I can double a smoker's chances of still being alive a year after a heart attack simply by encouraging him or her to stop. They assume that modern medicine will look after them, but our therapeutic armoury can never be a substitute for self help - it's we doctors and our drugs, not the lifestyle changes, that are the icing on the cake. If people want to engage in risky activities and ignore health education messages then that is their right, and they should be entitled to the very best medical care when things go wrong - as they inevitably will. But the priorities of healthcare are changing.