It's really just as simple as Morita decided to quit. He talked a big game about becoming a villain and surpassing Ginji but he just wasn't up for what that path entailed. In essence he had the talent to become the ruler of the underworld but not the heart. You look at guys like Ginji, Harada, and Akagi and you can see why guys like them thrive in the underworld. Now look at Morita since the beginning he didn't have that it factor to do it. Thinking back the biggest sign of this is his confrontation with his partner after the art dealer scam. With his partner showing and telling Morita what it takes to be a villain and denying it saying that he was wrong. In the end Morita reminds me of Hiroyuki, both of them tried to walk a path to be on the same level as the person they perceived as god-like(funny how they are both older men with white hair) but in the end realized they'd never be able to have what it takes to reach that level.