If cancer is not contagious through blood then why cant cancer patients donate blood ?

If the cancer is in remission, the person is in good health, with a normal hemoglobin, not taking any medications, and the person meets all other requirements to be a blood donor, one could donate. However, a patient with systemic cancer is unlikely to meet the requirements for donation. Visit this for info: https://www.redcrossblood.org/donate-blood/how-to-donate/eligibility-requirements.html

There have been instances of people who unknowingly had untreated cancer & donated blood with no instances of cancer transmission to the recipients. I'm sure that most clinicians don't believe that cancer can be transmitted by transfusions. You can understand that people want blood transfusions from only healthy donors.