There are a few possible explanations.

One is that the animals may just be better at preventing or dealing with cancer. We know the, for example, part of the hardiness of tardigrades comes from special proteins that are really good at repairing DNA. It could be that large animals have similar tools that we haven't identified yet.

An odd possibility is "supercancer". Cancer is deadly because as tumors grow they suck up vital resources from functional tissues and organs, and physically get in the way of the organs. What if a tumor had its own cancer? The secondary tumor would suck up resources and choke out the primary cancer, just like cancer normally does to healthy tissue and organs.

Because the animals are so big, tumors can grow for longer and get bigger than they could in smaller animals like humans. That gives more opportunities for supercancers to develop inside those tumors.