I think it’s mostly to do with their metabolism. (Metabolism is basically all of the chemical reactions and processes that go on inside us to keep us alive).
Animals with a very fast metabolism like rats and mice tend to only live a very short amount of time whereas animals with a very slow metabolism like tortoises tend to live for a long time (assuming they don’t get any diseases).
One possible reason for this is that our metabolism produces some harmful byproducts. We are pretty good at getting rid of these harmful products but I guess if your metabolism is slower then you produce these harmful products slower and so you have more chance of getting rid of them.
Perhaps the other scientists will have more ideas?