Rob Lowe departed the series after episode 17, saying he was not happy with his character Sam Seaborn and believed he did not fit in the show anymore.

Why did Rob Lowe leave The West Wing TV show?

Despite his charm and charisma, Sam was written out of the show in the fourth season as a result of disagreements over pay and the diminishing size of his role on the show.

Why did Rob Lowe get top billing on West Wing?

The opening credits, actually, tell much of the story: Lowe receives first billing, with others following alphabetically before the “and Martin Sheen” tag. That's because Lowe was the biggest name going in, the handsome onetime movie star who would help NBC promote a concept that made network executives antsy.