I would guess 40 and 50 years ago it was difficult to diagnose or find and if one were to die from cancer it was so metastasized by that time it was difficult to pin down where it actually was. Plus we live a lot longer and there are a lot more people. Just my guess.
actually I do not think that was the case. Teaching hosptials and post mortems were key then. There were a lot of cancers that were well known however were non treatable.