My Father was very careful about what he put in his body. Didn't drink, smoke...he didn't each much meat...everything in moderation, really. He died of Colon Cancer, a cancer they told us had been in his body for a very long time. He also had bladder cancer and prostate cancer. Sometimes I just think it's the luck of the draw.
I am sorry for the loss of your father. A reasonable diet and proper exercise does not cure anything, especially cancer. It does help us to resist diseases for a longer time, in some cases long enough to survive treatment. Realistically, it may be luck of the draw. But it is also important to have regular screening as well. Even if someone decides for themselves that they will not undergo treatment.