Steven Spielberg didn't direct Jurassic Park III despite directing the first two Jurassic Park movies, and that's because he had a promise to keep. Instead, Jurassic Park III was helmed by Joe Johnston, a filmmaker known for his work directing acclaimed blockbusters like The Rocketeer and later Captain America: The First Avenger. Unfortunately, Jurassic Park III, which starred Sam Neill and William H. Macy, ended up making considerably less than its predecessors when it released in 2001, though it still amassed $368.8 million at the worldwide box office.
Since Jurassic Park III, the Jurassic Park franchise has seen multiple non-Spielberg directors helming individual installments. Colin Trevorrow, for instance, directed Jurassic World and is directing next year's Jurassic World: Dominion. Meanwhile, the 2018 entry, Jurassic World: Fallen Kingdom, was entrusted to J.A. Bayona, who was the fourth director ever to direct a Jurassic Park movie. While Spielberg is responsible for making the Jurassic Park franchise special to so many people around the world, the series has since expanded from that vision to new regions. But in order for that to have happened, he needed to step aside in the late 1990s.
As noted in numerous interviews, including in ones seen on the Jurassic Park III home video release, director Joe Johnston expressed interest in directing a Jurassic Park sequel shortly after the first movie released. Though Spielberg wanted to direct the first Jurassic Park sequel, he promised Johnston that he would get his chance to direct dinosaur mayhem should Jurassic Park III ever become a reality. Once that very first Jurassic Park sequel, The Lost World: Jurassic Park, became a box office sensation with $618.6 million worldwide, Jurassic Park III was inevitable, as was Spielberg getting to fulfill his promise to Johnston.
It wasn't just a promise to Johnston that kept Spielberg out of the director's chair of Jurassic Park III, though. Spielberg's own personal creative ambitions ensured he wouldn't be back for the third installment. In the wake of his Best Picture-winning production Schindler's List, Spielberg was looking to pursue darker dramas rather than lighter blockbuster fare. This pursuit was reflected in how, in the 14 months after The Lost World, Spielberg helmed the weighty movies Amistad and Saving Private Ryan. Considering his artistic mindset at this point in time, Spielberg wasn't keen on directing another Jurassic Park movie.
Furthermore, Spielberg not directing Jurassic Park III was also influenced by the draining experience he had endured on the previous Jurassic Park movies. In an interview with Chicago Tribune timed to the release of Jurassic Park III, Spielberg stated a key reason he didn't return as director was because of how immensely difficult it was directing the first two installments. Directing such expansive blockbusters, ones that juggled animatronics, CGI, tropical weather, and so forth, it wasn't something Spielberg was looking to undertake again in the near future. Given all of these factors, it shouldn't be a surprise, then, that Steven Spielberg handed over the reins to Joe Johnston when it came time for Jurassic Park III.