Why did Marlon Brando not do Godfather 2?

Can you shed some light on the intriguing question of why Marlon Brando, the iconic actor renowned for his portrayal of Vito Corleone in The Godfather, chose not to reprise his role in the sequel, The Godfather Part II? Was it due to creative differences with the director or producers? Did he feel that his character's arc had been adequately explored in the first film? Or was there perhaps a personal reason behind his decision to step away from the franchise? It's fascinating to ponder the potential motivations behind such a significant decision in the history of cinema.