This isn't so precisely focused on tortoises, but a general theory in evolutionary biology for why some animals live longer is K vs r selection theory.
The idea here is that animals will make a sort of evolutionary 'choice' and configure themselves to breed as numerously and quickly as they can. This is called 'r' selection, named after the constant that is proportional to the rate or speed of breeding. r selected animals adapt quickly to a rapidly changing environment and their lives are pretty cheap. Many insects that lay hundreds or thousands of eggs every few weeks are examples of r selection. They do not care for their offspring (and may even eat them!).
K selected animals breed to be highly competitive in the environment. They tend to be highly optimized for success and this often means they are larger, they live longer and require more investment as newborns.
By this theory the galapagos tortioises live longer because their environment changes very slowly and the tortoises are so optimized for that particular way of life that they have no predation or any competition at all. Islands like the Galapagos are studied in fact because they are so isolated by the water that new species rarely ever arrive. Such new arrivals may cause drastic changes in their ecosystems because islands are stable environments because migration happens so rarely.
There are other theories that emphasize other factors in the evolution of larger, longer lived animals, but I think r/K selection may describe the environmental pressure on the Galapagos tortoise.