Those animals (e.g. tortoises, lobsters) with longer lives combined with less cancer have significantly lower metabolisms and a far less ... intense ... lifestyle.

Higher cellular respiration introduces higher chances for mutation / inflammation / cell damage. Likewise, high rates of cellular reproduction do the same. Every incidence of that is a roll of the dice for cancer. Extend the lifetime, and you will for sure get cancer, because you already have a very high chance of it.

Evolution in our species has arrived at a set of trade-offs. We don't understand them all yet.