Why did the harpsichord disappear from serenade ensembles in the 1700s?

The harpsichord disappeared from serenade ensembles in the mid-18th century primarily because it was considered unwieldy for the performance style of the time.1

As the custom of serenading became popular in Austrian and Italian cities, small groups of instrumentalists began performing in the streets. Because the harpsichord was used to realize the basso continuo—a practice requiring a keyboard instrument to improvise harmonies over a bass line—it proved impractical for these mobile, open-air performances.1

Consequently, the harpsichord was abandoned in these settings. To maintain the necessary harmonic structure, a viola was added to the existing string ensemble, and the bass line was reinforced by a cello, leading to the emergence of the string quartet as the standard medium for chamber music.1