It is a fundamental principle of evolution that things that replicate faster and survive better will prevail. We are made of coordinated, replicating cells. They happen to be in a mutual "truce" to act as one organism at the expense of individual cell survival. Most cells that break the truce do badly on their own and die out. However, given enough tries at the prize, some cells will find a way to beat the system and replicate at the expense of the other cells still serving the whole body.

Some people have genetic deficiencies that give potentially malignant cells a jump start on the process. Certain environmental exposures will increase the rate of errors and therefore the frequency of tries that cells will get to become cancerous. But the outcome will always be the same; as we prolong human life through better medical care the incidence of cancer can only rise.

It would be most accurate to say that cancer is an evolutionary phenomenon that occurs between cells of a multicellular organism and causes a heterogenous group of diseases.