Answer by taking a quote from the following article:

Stephen Joshua Sondheim (; born March 22, 1930) is an American composer and lyricist known for more than a half-century of contributions to musical theater. Sondheim has received an Academy Award, eight Tony Awards (more than any other composer, including a Special Tony Award for Lifetime Achievement in the Theatre), eight Grammy Awards, a Pulitzer Prize, a Laurence Olivier Award, and a 2015 Presidential Medal of Freedom. He has been described by Frank Rich of The New York Times as "now the greatest and perhaps best-known artist in the American musical theater." His best-known works as composer and lyricist include A Funny Thing Happened on the Way to the Forum, Company, Follies, A Little Night Music, Pacific Overtures, Sweeney Todd, Merrily We Roll

Burt Shevelove invited Sondheim to a party; Sondheim arrived before him, and knew no one else well. He saw a familiar face: Arthur Laurents, who had seen one of the auditions of Saturday Night, and they began talking. Laurents told him he was working on a musical version of Romeo and Juliet with Leonard Bernstein, but they needed a lyricist; Betty Comden and Adolph Green, who were supposed to write the lyrics, were under contract in Hollywood. He said that although he was not a big fan of Sondheim's music, he enjoyed the lyrics from Saturday Night and he could audition for Bernstein. Sondheim met Bernstein the following day, played for him and Bernstein said he would let him know. The composer wanted to write music and lyrics; after consulting with Hammerstein, Bernstein told Sondheim he could write music later. In 1957, West Side Story opened; directed by Jerome Robbins, it ran for 732 performances. Sondheim has expressed dissatisfaction with his lyrics, saying that they do not always fit the characters and are sometimes too consciously poetic. While Bernstein was working on Candide, Sondheim reportedly wrote some of West Side Story's music; Bernstein's co-lyricist credit disappeared from West Side Story during its tryout, possibly as a trade-off. Sondheim insisted that Bernstein told the producers to list him as the sole lyricist. He described the division of the royalties, saying that Bernstein received three percent and he received one percent. Bernstein suggested evening the percentage at two percent each, but Sondheim refused because he wanted the credit. Sondheim later said he wished "someone stuffed a handkerchief in my mouth because it would have been nice to get that extra percentage". After West Side Story opened, Shevelove lamented the lack of "low-brow comedy" on Broadway and mentioned a possible musical based on Plautus' Roman comedies. When Sondheim was interested in the idea he called a friend, Larry Gelbart, to co-write the script. The show went through a number of drafts, and was interrupted briefly by Sondheim's next project. In 1959, Sondheim was approached by Laurents and Robbins for a musical version of Gypsy Rose Lee's memoir after Irving Berlin and Cole Porter turned it down. Sondheim agreed, but Ethel Merman - cast as Mama Rose - had just finished Happy Hunting with an unknown composer (Harold Karr) and lyricist (Matt Dubey). Although Sondheim wanted to write the music and lyrics, Merman refused to let another first-time composer write for her and demanded that Jule Styne write the music. Sondheim, concerned that writing lyrics again would pigeonhole him as a lyricist, called his mentor for advice. Hammerstein told him he should take the job, because writing a vehicle for a star would be a good learning experience. Sondheim agreed; Gypsy opened on May 21, 1959, and ran for 702 performances. CANNOTANSWER

He was supposed to write the lyrics--what happened?

he was not a big fan of Sondheim's music, he enjoyed the lyrics