Why did Christoph Willibald Gluck oppose elaborate vocal ornamentation in opera?

Christoph Willibald Gluck opposed elaborate vocal ornamentation because he believed it hindered the "true office" of music, which he defined as "serving poetry".1

During the 18th century, the prevailing style of opera seria was often criticized for its reliance on coloratura—highly embellished vocal melodies—and other ornamental features that impeded the dramatic action. In the dedication to his opera Alceste, which served as a manifesto for his reform movement, Gluck argued that these "useless and superfluous ornaments" encumbered the music and distracted from the narrative.1

Instead, Gluck sought a "beautiful simplicity" and naturalness of expression, aiming to restore opera to its function as a drama set to music, guided by Enlightenment ideals. By stripping away these excesses, he focused on emotional truth and allowed the music to better support the dramatic strengths of the libretto.1