According to the ACS:
Breast cancer has the highest rate of new cases in the US and also has a survivability rate of about 5.5 to 1. Also, breast cancer not only has a significant health impact, but also a major physical impact for the rest of women's lives (even after survival)
Prostate cancer IS a close second in new cases in the US for new cases. However it's survivability rate is more around 8 to 1.
According to Me:
While neither of these have not even close to the highest mortality rates, these types of cancers are generally known to be some of the easiest to diagnose early and exponentially increase survivability when done so. (lets toss in Colon and Melanoma as well)
Therefore, one can suggest that maybe concentrating efforts and money on the most bang for the buck (so to speak) is the most beneficial path. To me, it's not about 'save the tatas' or 'save the pepes', it's about how to protect and save the most people possible. But, I'm certainly never going to complain or criticize anyone for donating to or concentrating (usually due to a personal impact) on any type of disease that effects humans (also throwing in animals, cause I'm am)