Helen Traubel elected not to renew her contract, which expired on May 30, 1956. May 30 is traditionally the official end of the New York theatre season, and at one time it was common for many Broadway contracts to expire on that date. It was reported in the New York Times on May 18 that one of the candidates to replace her was Dolores Gray, but it was Nancy Andrews who was ultimately hired to replace Traubel. By the time Andrews was hired, there was insufficent time for her to be ready to take over on June 1. Traubel agreed to stay till Andrews was ready. Andrews took over on June 11.
Understudy Ruth Kobart surely could have filled in, but Traubel had missed many performances during her time in the show, including all the performances from December 21, 1955, though January 10, 1956. (She was ill with bronchitis during that time.) So she may have felt a bit obligated to help out as a way of making up for the performances she had missed.