Has it occurred to anyone that the food itself is only partially to blame?

Besides genetics which dragondawg mentioned, there are way more environmental factors to consider.

Even people seem to be getting cancer more, people travel more and further (taking their pets with them) than they did in the 60's thus exposing their animals to more things or just having more diseases brought into their own backyard by traveling pets. Diagnostics have gotten better every year in both people and animals, it's possible that what used to be just lumped in one category, they now know what it is and have a name and separate category for it as well as tests to detect it.

Something to think about.