Why are organs not harvested from deceased cancer patients?
There are different reasons why cancer patients are mostly excluded from donating organs.

Although the probability of transmitting cancer is small, it is not zero. Also tumors tend to form metastases over time, which spread throughout the body. It is possible that a cancer has formed metastases in other, distant organs, which are too small to detect. Since recipients of organs have to take strong immune suppressing medication to prevent the rejection of the transplant, the immune system might not be able to fight the cancer cells anymore. Because of this medication, transplant patients already have a higher chance of getting cancer, because their immune system does not capture cells when they go rogue. See reference 1 for a more general description and 2 for the detailed report, reference 3 for the higher cancer risk of transplant patients.
Tumors can go dormant when they are treated with chemotherapeutics to evade destruction, but these dormant cells can re-wake. If this happens after the transplantation of an organ, this is obviously bad for the recipient. See reference 4 for details on mechanisms.
Cancer patients who died from the disease have often been treated with agressive drugs like chemotherapeutics. While these attack mostly the fast dividing cells of the tumor, they also have more or less strong side effects on the body. A lot of organs are simply not in the state (or have a strong chance of being in this state) anymore to be transplanted. Nobody wants to transplant a dysfunctional organ. See reference 5 for details.

References:

Cancer Spreads from Organ Donor to 4 People in 'Extraordinary' Case
Transmission of breast cancer by a single multiorgan donor to 4
transplant recipients
Cancer in the Transplant Recipient
Tumor dormancy as an alternative step in the development of
chemoresistance and metastasis - clinical implications
Long-Term Side Effects of Cancer Treatment