For evolution to happen a trait needs to effect reproductive success. Diseases that hit later in life will only have a minimal effect. Your kids are already gr

For evolution to happen a trait needs to effect reproductive success. Diseases that hit later in life will only have a minimal effect. Your kids are already grown by the time you get cancer and probably having children of their own. There may be slightly less resources available to raise the next generation if you die but the effect is minimal. Genes that help/hurt the survival of young people are going to have a much a more profound effect and influence evolution more.