A good way to think of this is that animals all live roughly the same average number of heart beats. Human hearts beat say 60 times a minute and we live say 70years so that is 60•60•24•365.25•70 which is like two billion or so. Now a humming bird only lives like two years so its heart beats 35times faster. Or a tortoise lives 210 years but its heart beats a third of the human rate.

What this is getting at is mostly metabolism. And metabolism has a lot to do with cell division. And cell division has a lot to do with cancer rates.

Edit: A good question to ask is shouldn’t a blue whale have more instances of cancer because it has more cells, but cancer is also tied into the age of cells in a poorly understood way.

That was the follow up question I was going to ask actually, about blue whales and how they have more cells which sensically would make you assume its easier for them to get cancer, but as you said, it's not very well researched. I saw a while ago a kurzgesagt video which I can't find which theorised that big animals such as whales get hypertumors often which kill their cancers and create a rate more even with the rest of us. Maybe that's why.

The short answer to your question is we simply don't know, and this is an area of ongoing research. We should expect to see cancer far more frequently in larger animals, just given the larger number of cells and the larger chance of accruing oncological mutations. Surprisingly, this is not what we see when we look at animals such as elephants and whales, and we really don't know why

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.

It's likely because the same basic processes that cause cancer in humans, like mutations, also occur in other animals.

It's likely because the same environmental and lifestyle factors that contribute to cancer in humans can also affect animals. Additionally, some animals may be more susceptible to certain types of cancer due to their genetics.