The main reason I'm given when new players leave is simply bored. As a new knight they often don't have the opportunity to engage in the politics and other factors which really make this game fun, they set up their estates, then wait for orders to click the travel button twice a day.
Which is why non-mentor older characters need to step up and send private messages to new characters inviting them into guilds, religions, helping them find lords, or engaging them in conspiratorial activities.
New players are ready-made political allies. That is what players need to realize. It's not really a question of benevolence, but of self-interest for powerful nobles. That has generally been my approach in Terran, and we've had a lot of new players come through for a realm our size and age and make it into upper positions. Many then leave or pause later on, but we still get a good few months, and some stay. Current examples would be the Van Kaya family, the Ryngaar family, the Maldives family, the Somnius family, and others that are now gone.
Indeed, that is my own story. I joined and left after 3 days, then joined again a week later and stuck it out for a while. Got involved in a conspiracy in Sartania, got booted out. Randomly went to East Island. There found Paul Keithson, one of Marc J's characters, and that is basically why I stayed with Battlemaster, because an established noble took Hireshmont under his wing and gave him a role.