Personality, principles. Diocletian stepped down as Roman Emperor and went to grow cabbages. Lucius Cincinnatus relinquished dictatorial powers at the height of his popularity and went back to his farm. Pope Celestine V was a hermit living in a cave who sent a scalding letter to the cardinals in Rome for ther inability to elect a pope for 2 years. The cardinals elected him. He became the most powerful person in the world overnight, and he used that power to issue a decree that allowed for the resignation of the pope and immediately used it. He was pope for 6 months. Some people have the wisdom and integrity to treat even limitless power as a tool, and may reach a point in life where they no longer feel the need for that tool. You're right that it's not common though.
Personal trauma could be a thing also. Imaginne, you fail to save someone you love, and suddenly all magic seems stupid. Sure, you can conjure fireballs, or castles, or whatever, but to what end? It wasn't enough when it mattered, and no magic you can hope to do will undo that. The mightiest spells might feel like useless tricks. You may keep using spells to keep you warm and fed, but the idea of poring over tomes year after years just feels hollow.
Limiting your power is kinda house culture for Jerbitons. Many of them believe that people who devote all of their lives to perfecting spells and developing Arts are effectively crazy. House Culture has this idea of "good enough". They tend to respect having a solid command of your Arts and using them with some sense of purpose, but they shun hyper-specialized experts.
Religion could also explain some... apprehension, when it comes to accumulating more and more sorcerous power. Like, there is not a whole lot about magicians in the Scripture, and what is written does not speak very highly of magic. Maybe you're going to have uncomfortable moments on the day of Judgement if you've spent your whole life amassing destructive or mind-bending power?
Are there Magi of other Mystery Cults who have renounced their Mystery Cults? Verditus, Bjornaer, Merinita, etc
In the Saga I SG I used an ancient Bjornaer known as Hellerzahn whose Heartbeast is a European Viper, and who lives in the wilderness. She has been in an Epitome form for over fifty years. She did not quite renounce power, but took an avenue of research based on studying and perfecting the Epitome to a point where she is difficult to even communicate with. Functionally no longer a Bjornaer, she does not go to Tribunals, Gatherings, etc. Most think she is in Final Twilight.