What the previous comment says makes sense, but I think economic and creative issues carry more weight than it seems. An actor who became famous for an iconic role often finds himself trapped: studios want him back because they know it works at the box office, but he's not the same person he was a decade ago and probably wants to explore different characters, not stay pigeonholed. Plus, conflicts with producers and directors also play an important role - if the first experience was toxic or complicated, why go back? And yeah, salary differences matter too; a lot of times they offer him less than he deserves because they think "it's his most famous role, he should be grateful." In the end, turning down a sequel is an act of control over his own career, not just artistic whimsy.