I was assuming that elephants lived longer than that, but according to a quick google search, Asian Elephants tend to live for ~50 years. This elephant lived a good long life! land creatures have much more stress on their body due to gravity, it’s why larger dogs tend to live shorter, and larger animals tend to live shorter than their smaller counterparts.
If you’re talking about invertebrates, because of the higher concentration of oxygen in the prehistoric atmosphere. Invertebrates get oxygen in their body by diffusion. The higher partial pressure of oxygen means it can get deeper. If you’re talking about fish, selection pressure. Bigger fish got caught more often by humans, so it was an advantage to be smaller. They didn’t get caught or were thrown back and got to reproduce.