Why is cancer not a communicable disease?
A cancer is not a pathogen

Cancer is a group of cells that (because of several specific mutations) start to duplicate abnormally. This group of cancerous cells are the own cells of the sick patient. It is not another species infecting the individual carrying the disease. It is the individual itself who developed the disease. Cancer is not a pathogen (but see below) that infect hosts and get transmitted from one host to another. It is more correct to picture cancer as an internal mistake of the body rather than as an external attack on our body. You might want to have a look to the wiki page for cancer for more information.

Transmissible agents that can cause cancer

Some cancers are caused by transmissible agents, such as oncoviruses. the transmissible agent may jump from one host to another and cause cancers giving the sensation that cancer is transmitted. It is just a sensation of course because in reality only the oncovirus gets transmitted and not cancer itself jumping from one host to another.

Cancers that become transmissible

Note however that it is not impossible to imagine a tumour jumping from one individual to another in cases where the immune response cannot detect cancer as 'non-self'. Some cancers are transmissible. Those cancers are called clonally transmissible cancer (As indicated by C Rags in the comments). Here is the list:


The Devil Facial Tumor Disease which occurs in Tasmanian Devils
The canine transmissible venereal tumour (article from Chris Stronks in the comments)
The Contagious reticulum cell sarcoma occurring in the hamster.