Because cancer is caused by changes, (mutations) to the DNA within cells, (The DNA inside a cell is packaged into a large number of individual genes each of which contains a set of instructions telling the cell what functions to perform (and how to grow and divide)) the changes we have or are occurring is often not in recollection to what changes are actually being made to our body, therefore, making it more of a possibility to cancer. (Cancerous)
Hope this helps! :)