Well, the filmmakers had their own vision for the cast. They chose actors like Uma Thurman, Bruce Willis etc. who fit the style of the movie, and De Niro just wasn't part of that plan.
One example is 'Eternals' where he played a role. In this movie, the science - fiction elements are quite prominent, with the story revolving around a group of immortals.
Robert De Niro has no part in Pulp Fiction. Pulp Fiction is a cult classic filled with Tarantino's signature style, from its nonlinear narrative to its pop - culture - laden dialogue. While De Niro is a highly respected actor in Hollywood, his work lies in other movies. For example, he has been in Scorsese's films such as Taxi Driver and Goodfellas, which are very different in tone and style from Pulp Fiction.