That's because most Radiation Oncologists do not participate in designing clinical trials, performing bench research, etc.
In fact a sizable number of MD/PhDs who go into RadOnc end up going into private practice like all the rest. You are right in that academic programs are "gung ho" about MD/PhDs. Ostensibly they do it to produce academic physicians but sometimes they don't give residents the time they need to pursue research. Thus recruiting a large number of PhDs becomes a mark of distinction more than anything else.
However, even in academics I have found that starting salaries for radiation oncologists are on par (and in some case, slightly higher) than private practice. The problem is that your salary will not go very high after that whereas in private practice it will take a large bump when you make partner and can continue to rise with high patient volume.