Why did George Gershwin improvise part of the piano solo at the premiere of Rhapsody in Blue?

George Gershwin improvised much of the piano solo during the premiere of Rhapsody in Blue because the work was written in great haste and remained somewhat unfinished by the time of the performance.1

While Gershwin and his arranger, Ferde Grofé, successfully prepared the orchestral parts in time for the concert, the solo piano part had not yet been committed to paper. Consequently, the solo existed only in the composer's mind, and Gershwin performed it from memory at the premiere. Because of this, conductor Paul Whiteman had to rely on a nod from Gershwin to cue the orchestra at the conclusion of the solo.12